There was a single cause. Europe was Broke and needed a come up.This is revisionist history. There was no single cause for the crusades as a whole, because not all the crusades were the same. Western Christians sacked, looted, and devastated entire Eastern Christian kingdoms, and Jews, too, not to mention each other, through the crusades, and same goes for the Muslims, who were by no means a monolithic mass of united people.
Taxing donations is retarded though, will cause churchs to close and will prepare the people to accept more debauchery in legislation over the decades.