Hey! I know this!

The Thirty Years' War was an incredibly complex political event, so this is by no means an exhaustive answer. That being said, I can give you a couple of reasons that certainly contributed towards the huge supply of Scottish mercenaries during the war.

First off, Scottish motivation to become mercenaries during the war is intrinsically linked to overall British motivation, the most prominent being that of the 17th century political scene. In 1613, Elizabeth Stuart was approached to marry Ferdinand II, the elector Palatinite, which would in turn grant Elizabeth the title of the Queen of Bohemia. This marriage was met with great public support and enthusiasm, and thus the relaitonship between Britain and Bohemia was one of great national pride. However, the outbreak of the war led to what is contemporarily known as The Bohemian Phase. Following Bohemia's revolt, Elizabeth Stuart faced a possible loss of her lands in the upper and lower Palatinite. This, along with Britain's new agreement with the Danish (known as The Treaty of Hague) would, therefore, demand British participation in the conflict. James V would immediately deploy a regiment of 200 Scots to serve with the Danes in Bohemia against the Catholics, among which being Colonel Seton. This is where we can start to analyze if this political motivation was really shared by the troops, or merely that of British politicians. Seton, during his ventures, would be unsuccessful and face a number of losses during this period, with many of his fellow Scots surrendering following disasterous military sieges and skirmishes during this Bohemian phase. However, after being surrounded by the Imperial army, Seton was awarded the opportunity to surrender himself and join his fellow Scots. To this, Seton responded with disappointment that the other Danish and Swede forces had surrendered but merely responded that he would not sacrifice his loyalty to the Stuart dynasty.

Another case-study we can look at is the career of Robert Monro, a now mildly famous figure among Scottish military historians. Monro kept a consistent diary during his career as a Scottish mercenary in The Thirty Years' War, and wrote his account as an almost text-book of sorts for which to describe the motivations of Scottish soldiers. Monro, amongst other Scottish mercenaries, refer to Elizabeth Stuart as the "Jewelle of Europe", a term used in great pride by the Scottish mercenaries when asked to identify their motivations for joining the conflict. Indeed, Monro's loyalty to Elizabeth was clearly easily identifiable as she would, herself, write a letter to other Scottish mercenaries asking them to heed Monro's lead, due to his loyalty to the Queen. However, this is not to say all soldiers were motivated by this loyalty, as we see during the Seton example that other Scottish soldiers did surrender, and Monro did often face unsuccessful revolts by his fellow soldiers. That being said, we have very little writings from lesser soldiers, thus making the picture somewhat blurry. Evidently, however, we can say that a number of Scottish officers were motivated by loyalty to Elizabeth Stuart.

Another fact which naturally attaches itself to the career of a mercenary is, of course, economic reasons. Economic necessity most certainly motivated Scottish troops to join, and soldiers would often refute their loyalty if they were not given sufficient payment. Monro, despite having more fundamental reasons for joining, acknowledges this as a fact for the enlistment of soldiers, as he writes that his cousin (also named Robert) joined due to being an "excessive spender" and was thus recommended into military service by his family. Whilst fighting in the war, plundering was extremely common and was evidently participated in by Scottish troops, as James King writes jokingly that mercenary work is "more akin to voluntary work", due to the failure of the military and the administration to pay their troops. One such instance of this is seen in the regiment of Charles Morgan (who did come into contact with Monro's regiment whilst under Danish service). The economic necessity of Morgan's men is seen as, whilst they had been assigned with the mission of delivering an expensive jewel to Christian II of Denmark, the soldiers would pawn the jewel off in exchange for payment (at a price lower than its initial valuation). Furthermore, Morgan's men would repeatedly threaten him with mutiny due to a complete lack of payment, and Morgan would have to travel to area's such as Hamburg to get in touch with Scottish administrative figures, such as Robert Anstruther, in order to secure payment for his regiment. Lest his soldier's revolt against him.

If you're interested to learn more, I can give another two reasons of similar length regarding religious, 'virtuous' and moral motivations! These are, however, two of the most straight-forward motivations.

Sources:

S. Murdoch, Scotland and the Thirty Years War, (Brill; 2001, The Netherlands)

R. Monro, Monro, His Expedition with the Worthy Scots Regiment called Mac-Keys, (Praegar; 1999, USA)

E. A. Bellar, 'The Military Expedition of Sir Charles Morgan to Germany, 1627-9' English Historical Review (1928) pp. 528 - 39