While I suspect there isn't a single, specific to this question, I can hopefully point you in the direction of some contributing factors.

Firstly, the nature of the Scottish kingdom/state in the medieval/early modern period. Scotland, much like England, had its nominal first king in the ninth century, but this position never really attained the level of authority and centralisation that England did (and England obviously also still had plenty of civil wars/coups/rebellions during the middle ages). From the outset, the Kingship of Scotland was closer to the title of a warlord than a traditional monarch, and frequently switched hands based on war or assassination. As other Europeans kingdoms were gradually moving towards limited forms of centralisation in the late middle ages, Scotland was rocked by English invasions and only just managed to continue to exist as an independent kingdom. That Robert the Bruce personally had to assassinate a rival to the throne before leading the Scots to victory over the English sums up how violent and precarious a position King of the Scots was.

Part of the reason for this is that even by the standards of the loosely centralised states of the medieval period, Scotland was not so much a cohesive kingdom as it was a collection of aristocratic, regional and tribal interests, which were often in open or simmering conflict with one another and which no one king was ever likely to placate all of. Geography was key, and Anglicised lowland lords often had little in common politically or culturally with their Gaelic subjects in the Highlands and Isles (a situation complicated further by the fact that parts of the Isles were controlled by Norwegian lords until well into the middle ages). The clan system was still extremely strong, and would intersect in extremely influential ways with the 'high' politics of London and Edinburgh until the dawn of the modern era, most famously in events like the Glencoe Massacre of 1692 or the Jacobite Rebellion of 1745. The upshot of this was that kings were dependent on maintaining the loyalty of a sufficient power-base in a very transactional fashion. By the 15th century, where your question comes in, this had been formalised into a uniquely Scottish, transactional form of feudalism known as 'manrent'.

During and after the Reformation, these bonds of manrent would be re-imagined as 'covenants' and 'confessions' which were documents, in effect treaties, which bound the monarch to defend certain patterns of religion and politics. While the English Reformation is generally remembered as a watershed of centralisation and royal power, the Scottish Reformation was, in many ways, just the opposite. It was driven by the aristocracy, and in effect underlined and confirmed that the position of King of Scotland was heavily dependent on the goodwill of his vassals, but actually bolstered this de facto situation further by producing a body of what became known as 'resistance theory' from Protestant writers like John Knox and George Buchanan who set out limits for kingly power and situations where it was legitimate to resist it. Such was the decentralised nature of the Scottish state on the eve of Reformation, that the Scottish historian Arthur H. Williamson has argued that the Reformation essentially required the wholesale creation of national institutions. The Reformation period, much like earlier ones, also featured a healthy amount of interference from England, which further undermined any prospect of stability in Scotland.

Following the Union of Crowns, Scottish ideas of limited kingship would be deployed to resist English impositions on religion in 1638 via the 'National Covenant' and essentially reconstitute an independent Scottish state which, while professing nominal loyalty to the king, would openly go to war with him in 1640 and 1644. This event was instrumental in sparking what is known as the English Civil War, but included both Scotland and Ireland. This is rather a separate issue to your question but its illustrative of how weak the institution of Scottish monarchy was that after the Union of Crowns, it could be argued that its weakness actually undermined the monarchy of England as well.

One thing to keep in mind when it comes to the Medieval period is that we are not really dealing with "fixed states". The nations we know today are not formed yet, even if some of the polities may carry very familiar names. At this time, there was nothing to indicate that "Scotland" would one day hold its present lines, regardless of Royal ambition to that effect.

The Irish Sea World gives an alternate view of how power could be organized, creating an axis between Western Scotland and the Isles, Norway, Ireland and England. In fact, earlier in the Medieval period, Western Scotland had strong ties with Ireland via the MacDonald Lordship, that eventually held the Earldom of Ross in Scotland and Antrim in Ireland. The Norwegian ties are better known, of course, with Viking raiders beginning to move south into what are today called the Hebrides before the year 1000. People of mixed Gaelic and Norse lineage became known as the Gall-goidheal, or Norse-Gaels, and the MacDonald Lordship of the Isles could trace its lineage back through Somhairle, the first Norse-Gael King of the Isles. Norway was also officially entitled to a payment from Scotland for the Hebrides from the time of the Treaty of Perth in 1266 until James I ended it. This means that any centralizing King of Scotland would face in the Western Isles a formidable dynasty with a well-established power base.

To begin at the beginning, somewhat, would mean starting back with David II, and the Stewart acquisition of power through Marjorie, sister of David and mother to the man who would become Robert II. It was not an uncontested succession, as the actual rules around succession in Scotland at that time were not concrete and David II had attempted to change them several times before his sudden death. In any case, Robert II was successful in acquiring the crown, and in feudal tradition made his eldest son John his heir. However, Robert II’s policy to strengthen Stewart control of Scotland via his sons was undermined by the brutality and aggression of his third son Alexander of Badenoch, also known as the Wolf of Badenoch. While supporting his father’s aims of extending Stewart power in Scotland, Alexander’s methods were met with widespread disapproval. He also used primarily Gaelic-speaking caterans from his lands Ross to wage a violent campaign against his enemies—the general bias against the Gaels becomes stronger at this time as a result.

A small aside, it is important to note that Gaelic was not yet a minority language in Scotland, and the Stewart kings did speak it up to James IV, and were very closely tied to Gaelic speaking lands and lordships.

Robert II’s unwillingness or inability to control Alexander saw his kingship undermined by his council and effective power handed to his eldest son, John of Carrick (later Alexander III) in 1384. John did attempt to reign in his younger brother, but was considered relatively unsuccessful. John was wounded at the Battle of Otterburn, where his ally the Earl of Douglas was slain (small aside, this is also known as the Battle of the Cheviot Chase, which is where Chevy Chase took his stage name). Douglas had died without issue, and John and his brother the Earl of Fife backed different claimants; Fife was ultimately successful and was able to use this new allyship to oust John from the Guardianship in 1388.

That means, when Robert II finally died in 1390 and the kingship passed formally, John (now officially crowned as Robert III) had to wrest effective power from his brother the Earl of Fife. Primogeniture as a means of succession was also relatively new in Scotland, being part of David II’s feudalistic reforms, so Robert III came to a vastly weakened kingship. Charges of lawlessness in the north were once again leveled, and Robert III once again found himself without power in favour of his brother Rothesay, who supported the Wolf of Badenoch’s depredations. They came into conflict with Donald of Islay, Lord of the Isles and in fact their cousin, whose lands bordered Alexander’s, but this limited conflict did not come to a head at this point. Rothesay began acting with increasing independence, but died before he could take power more formally.

The guardianship then passed to Albany, another brother of the putative King Robert III, and Robert took steps to ensure his minor son James could be moved out of the country and out of Albany’s reach for fear of an assassination attempt. However, James’ ship was captured by pirates and he was delivered as a hostage to King Henry V of England. That left Albany as ostensible guardian of Scotland following Robert III’s death in 1406 during James’ captivity. However, Murdoch Stewart, now Duke of Albany showed little interest in retrieving the King his cousin, and instead continued his policy of extending his own power into new parts of Scotland. He came into conflict again with Donald of Islay over the Earldom of Ross, the richest earldom in Scotland, to which both laid claim. Albany claimed it as guardian of the surviving heiress of Ross, and Donald as her uncle by marriage (of note, claims through marriage were acceptable at this time). It came to a head at the Battle of Harlaw, which is often used as evidence of longstanding enmity of the Gaelic Lordship against the Crown, and even the last stand of Gaelic independence. However, note that Donald certainly had eyes for Ross in mainland Scotland, was descended from Scottish Kings, and was rebelling against a rather questionable guardianship rather than the Crown itself.

There is some evidence that Donald was in communication with James at the time of Harlaw, but James was not released from English captivity until 1425, when he unleashed a vicious campaign against his cousin and ultimately executing Murdoch and two of his sons for treason. One son, James the Fat, escaped to Ireland, and the powerful Albany Stewart family was all but destroyed. James I was now relatively secure in his kingship, and set about establishing power in parts of Scotland that had not felt Royal interest for years. Although he initially did affirm Donald of Islay as Earl of Ross, he later moved against Donald’s son Alexander and attempted to divide the enormous MacDonald holdings. This comes into play in the later instability of his son’s rule.

The sons of Robert II were not done with their quest for the kingship, however, and Robert II’s youngest son Walter had been created Duke of Atholl under Albany’s guardianship. A later falling out between the two lead James I to ally with Atholl against Albany and grant considerable favours to him. However, Atholl’s son was second in line to the throne after James’ own son, a mere child. He recruited former Albany supporters from the King’s household and together they formed a conspiracy against the King, which resulted in his murder. However, the Queen managed to escape with the young prince. Supporters of the King were able to capture Atholl’s heir, and he was executed the day after the young prince was crowned James II.

There now enters the background of the guardianship of James II, which is fairly complex in and of itself. A notable factor here is that event had not only left the throne of Scotland in a minority situation, but left only a handful of earls of legal age in the entire country. Of these, James II’s uncle the Earl of Douglas, his distant cousin Alexander of the Isles and Earl of Ross, and the Earl of Crawford. These three were suspected of entering a tripartite agreement to support each other’s independence from the Crown. Due to his support for the widowed Queen, the Earl of Douglas was made guardian of the child-king. However, his sudden death left a power struggle between the new Earl of Douglas and the Chamberlain, Richard Livingstone, who may have feared a resurgence of Albany support from the Queen’s new husband, a Stewart of royal decent. Whatever his motives, though, Livingstone acted to kidnap the Queen and her new husband and demand custody of the young king James II. He later had the young Earl of Douglas and his brother, the king’s cousins, invited to dinner, where they were murdered, thus securing Livingstone’s influence over the king. You may recognize here the inspiration for George R.R. Martin’s Red Wedding.

When the King came of age, the new Earl of Douglas, William, urged the king to take revenge against the Livingstones for the death of his Douglas cousins (an affair in which William’s father James the Gross certainly had a role and possibly William himself) and the kidnapping of the Queen. James II did in fact move against the Livingstones, which also put him at odds with the Earl of Ross, now John II MacDonald of the Isles. James also attempted to force the Earl of Douglas to renounce the alleged alliance that continued between him and the now out-of-favour Earl of Ross. This alliance would have represented a very powerful force allied against the king, so the future stability of his rule could be called into question. When Douglas refused, the King murdered him. Ross promptly rebelled, although it is impossible to say whether it was in response to Douglas’ death or the move against the Livingstones; the Auchinleck Chronicle is the only surviving account of the event, and the date given for the rebellion is unclear. The King then faced a slow moving civil war in support of the Douglases, with their power finally broken in 1455. An unfortunate cannon explosion killed the king a mere five years later, leaving yet another minority rule for James III, though over a Scotland far more united under central power.