Christopher Tyerman outlines a range of motives behind the First Crusade
Urban II, who was Pope from 1088 to 1099, had a number of different motives for calling the First Crusade. One was, he was asked by the Emperor of Eastern Christianity – the Byzantine Emperor Alexios I – he was asked to raise troops to defend Eastern Christians from Turkish invasions. That’s one political reason. Urban himself had another reason; he was fighting a war of survival against a rival pope, and to portray himself as the leader of Christendom by raising troops and defending Eastern Christendom was obviously a big bonus, and it allowed him to assert a great deal of political clout in Western Christendom.
Also, there’s a third point – that Urban was very conscious of, in the 11th century, of Christians regaining lands that had been lost to Muslim invaders centuries before. You can see this in Spain, you can see this in Sicily before the First Crusade. So you have a combination of circumstances.