It is often said that attacks on pilgrims is one of the reasons the crusades happened

I'm not really sure who often says this. Not anyone I've read. There are some account of Urban II's call for the First Crusade at Clairmont which are fairly gory, but none of these are generally regarded as true.

I assume you mean attacks by Muslims on Christian pilgrims specifically because they were Christian Pilgrims, and not generic bandit raids and the like. The only example I can think of for this kind of attack is in 1009 when the "mad" Fatimid caliph Al-Hakim bi-Amr Allah ordered the destruction of the Church of the Holy Sepulcher in Jerusalem.

Pilgrims were generally a good thing for business, and both Christians and Jews were tolerated and taxed as second-class citizens under Muslim rulers, so there was no particular reason for those rulers to discourage the pilgrims.