I'll leave everyone with what I've learned concerning crusades.

While playing most Catholic factions, his Holiness seems to call most crusades against one of the Muslim nations, usually as part of the historical crusades, or at least that's how it seems.

Other times, he seems to call them, quickly, against a recently excommed Catholic faction, especially if it's still aggressive after being excommed.

He also seems to call them against Orthodox factions, if they get large, and are eating up Catholic faction.

Playing as both the Almo's and Eggies I have had crusades call on me out of the blue, as I was staying away from the Catholic nations. My guess was I grew to big, and the Pope felt threatened. I've also had them called on me after I'd wipe out about 1-2 catholic factions, and these were usually the small ones. Again my guess is I made the Pope not feel good.

So for what I've said, and know, the Pope calling crusade seems to be a fickle thing, and I never listen to the Pope, as doing them never seems to change anything between you and him, unless the crusade can help you with GA points, if that's required to get them.