Basically. whenever a vassal rebels it is for seniority. Why do they have such an incredible predisposition to it? I can't even figure how it even helps them.
Do vassals not usually try to institute whatever succession favours them most? I wonder if its possible that the longer potential lifespans might cause seniority to be seen as more preferable.
In the regular game, the AI logically goes for elective monarchy so they have a chance of inheriting. In Elder Kings, they seem to go for seniority instead, which makes no sense.