In my current game the Orthodox Greek lords of the Byzantine Empire decided to elect the Catholic Norman King of Sicily as the new emperor. It seems weird to me that they would elect a ruler who is both a different faith and culture rather than one of their own. I'm not exactly sure how the AI selects who its going to rally behind in Elective monarchy when its unhappy with the current ruler's line, but I feel their should definitely be a strong preference for rulers that share the AI's faith.