Best guess, he converted to his capital province's denomination. Check his wives, too: one or more might be Sunni, in which case he converted to please her.I recently started a Shia-game (CM, ruler designer, ironman) > the Khomeini-dynasty, starting of in Qom. The rest will be history...
Abbassids are starting to crumble, allready there is a Shia Caliph in Syria... things are looking up. Only one problem, for some reason my (fourth) character suddenly turned Sunni! My entire family is Shia, so in the end it's no biggie, but how the hell did that happen? I did give Sadaqah one time... anyway.