This is just a guess but i believe this is what is happening.Can someone explain to me how kingdoms decide who to declare war on? In all of my recent campaigns there are rarely any wars that aren't against the kingdom I'm part of. My kingdom is usually the most powerful, I know that has something to do with it but other powerful kingdoms avoid war with much more vulnerable neighbouring factions rather than taking advantage, so they don't grow to rival my factions power. If I use console commands to instigate wars they make peace straight way. Is it that they actually prefer to declare war on the kingdom the player is a member of? This is with the main quests disabled.
The AI kingdoms are always looking to improve their tribute status. What is hard to notice is that AI kingdoms actually make these very quick wars (1 day long) that are much more like tribute negotiations. It often happens when a kingdom is at war, others see that they would be weak if they also declared war on that kingdom (since it would be a 2v1) so they declare war to get a better tribute payment and make peace again (they like to stay in 1 war, unless very strong).
The problem right now is that I think AI kingdoms are doing the same thing to the player kingdom, but because they have no way to offer peace you end up in a bunch of wars that depend on you paying a losing war tribute (tribute are different for each kingdom, as offering peace is admitting defeat so is the worst situation for each kingdom, i.e. its a better deal for you to get them to offer peace except they can't right now). Your kingdom will eventually vote to get out of multiple wars but it will take much longer if you are the strongest faction as your AI lords wont want peace since they think they can beat the other factions.