It really should be a bit easier to predict which faction is going to secede. I had Averland try to secede out of nowhere. My relations with Averland were not great but it was positive and they were at 5 fealty. Secession should only be an option when the fealty is low (1 or 2) or when the relations with the faction are bad (-40 or something). Right now it's just way to random.
"The dog is a peasant and the cat is a gentleman." H.P. Lovecraft