I would say neither.
There were numerous events leading up to the crusades, but they were not inevitable.
Romanos Diogenes could have been a more effective leader in the leadup to the Battle of Manzikert, either at the tactical level (fighting in the Manzikert campaign and winning) or at the strategic level (possibly avoiding direct conflict and respecting the treaty with the Turks while strengthening Byzantine presence in the area) possibly leaving the Byzantine Empire in a better situation to the point where the emperor felt no need to appeal for Western aid.
Perhaps if Alp Arslan and the Turks had proven extraordinarily effective perhaps allying with Bulgarian rebels, they could have pressed their advantage to the point where they conquered the Byzantine Empire entirely, reduced it to vassal status, or placed some sort of friendly client-basileus on the Byzantine throne, preventing them from appealing to the West.
Pope Urban could have ignored Alexios Komnenos's call for aid, or put an unpalatable condition on it, like ending the schism and putting the Byzantine patriarchy under the papacy, thus depriving the crusade of papal sanction.
Urban's appeal for a crusade could have been ignored by Christendom's princes, leading to it either going nowhere or perhaps being answered by a few ill-prepared fanatics who would have been massacred at Civetot without any followup.
Pope Urban was a very important player, but not the only one. That doesn't make the crusades inevitable, though, because it just means that there were that many more places where history could have taken a different turn.
It's also possible that the crusades (or something like them) could have happened without Pope Urban, but that doesn't make them inevitable.
Like /u/Maklodes, I too have to answer neither. Sidestepping the philosophical debate surrounding inevitability, suffice to say that most historians do not see anything inevitable in the past, that a given event or series of occurrences was bound to happen in any meaningful way. Among other reasons, it lends itself to a teleological understanding of history where the past is rendered subservient to a specific end (in this case, Crusade).
However, it would be just as untrue to credit Urban II with the phenomena we know as the Crusades, or at least the big 'uns of the 11th-13th centuries. Sacred violence and other central tenets of crusading ideology have precedents throughout the early Middle Ages, and the events and processes leading up to Urban’s famous speech at Clermont in 1095 are not without doctrinal precedent. Perhaps it would be best to understand the events following 1095 as a synthesis of social, ideological, and doctrinal trends within the historical context in which they occurred.
Armed Crusade
Perhaps a good place to start in our understanding of the ideological origins of crusade is to throw out some of our terminology. Urban’s speech kicks off what is commonly called the First Crusade. This is a relatively late designator; few contemporaries of what we now call the First Crusade would have recognized it as such. Terminology associated with crusading tends to center around words like peregrenatio (pilgrimage), iter (journey), and cruciata (the past participle of cruciare, ‘to mark with a cross'). For the purpose of my response, I suggest that an interpretation of crusade within the framework and tradition of 'armed pilgrimage' lends to a better understanding of the roots of crusading ideology.
The reality is people in 1095 and subsequent years would have viewed the large-scale movements of Christians to the East within the context of a long tradition of pilgrimage to the Holy Land. Pilgrimage to Jerusalem was a fairly consistent feature throughout the medieval period and in the years approaching and following the year 1000, reports of large-scale pilgrimages to Jerusalem pepper the accounts of medieval chroniclers and writers. Rudolfus Glaber recalls in his Miracles de Saint-Benoit that at this time:
so innumerable a multitude began to flock from all parts of the world to the sepulchre of our Saviour at Jerusalem, as no man could before have expected; for the lower orders of people led the way, after whom came those of middle rank, and then all the greatest kings and counts and bishops; lastly (a thing which had never come, to pass before), many noble ladies and poorer women journeyed thither… Moreover, some of those were then most concerned in these matters, being consulted by many concerning the signification of this concourse to Jerusalem, greater than the past age had ever heard of, answered with some caution that it portended no other than advent of that reprobate Antichrist, whose coming at the of this World is prophesied in Holy Scripture.
Pilgrimages to Jerusalem also feature heavily in Glaber’s Five Histories, and contextually, both highlight the author’s preoccupation with what he believed to be the imminent arrival of the Antichrist- the large-scale pilgrimages being an obvious sign of the end times. Apocalyptic sentiments were a fairly regular feature throughout the medieval period, and the language used to reflect this preoccupation can be seen later in the various accounts of Urban’s speech at Clermont (though of course, being written with the successful conquest of Jerusalem in hindsight).
Furthermore, that those undertaking pilgrimage would be armed would not have been a radical departure of the status quo and the call to reclaim the Holy Sepulchre would not have sent shock waves when Urban II exhorted the princes and nobles of Europe to take up the sword and the cross. The road East was a long and perilous one with many hazards along the way, and the image of the suffering pilgrim, the object of violence, meshed well with apocalyptic readings of the church beset by violence. That is not to say that these large groups of pilgrims were expected to go silent to their martyrdom. In his account of the Great German Pilgrimage of 1064-1065 –a pilgrimage of several thousand led by several bishops, including Archbishop Siegfried of Mainz-, the annalist of Nieder-Altaich describes at length the violence encountered by the pilgrims, who were forced to take up the sword in their quest to reach Jerusalem:
On the following day, Good Friday [March 25. 1065] about the second hour of the day, just as they were leaving Kafar Sallam, they suddenly fell into the hands of the Arabs who leaped on them like famished wolves on long awaited prey. They slaughtered the first pilgrims pitiably, tearing them to pieces. At first our people tried to fight back, but they were quickly forced, as poor men, to take refuge in the village. After they had fled, who can explain in words how many men were killed there, how many types of death there were, or how much calamity and grief there was? Bishop William of Utrecht, badly wounded and stripped of his clothes, was left lying on the ground with many others to die a miserable death. The three remaining bishops, together with a considerable crowd of various kinds of people, occupied a certain walled building with two stone towers. Here they prepared to defend themselves, so long as God allowed it.
The gate of the building was extremely narrow and, since the enemy was so close, they could not unload the packs carried by their horses. They lost, therefore, their horses and mules and everything that the animals were carrying. The enemy divided these things among themselves and soon hastened to destroy the owners of the wealth. The pilgrims, on the other hand, decided to take up arms" and with weapons in hand they courageously fought back. The enemy more indignant than ever, pressed the attack more vigorously, for they saw that the pilgrims, who they had thought would not attempt anything against them, were resisting manfully. For three whole days both sides fought with full force. Our men, though handicapped by hunger, thirst, and lack of sleep, were fighting for their salvation and their lives. The enemy gnashed their teeth like ravening wolves, since it seemed that they were not to be allowed to swallow the prey which they bad grasped in their jaws.
Eventually, the pilgrims were able to negotiate a truce and after a few more hi-jinks and with the help governor of Ramla, were eventually able to reach the holy city on April 12, their victory after a long battle to achieve their goal.
hi! fyi, you'll find some information to get you started in the FAQ