I have this exact same problem! I'll defeat an army and before I can turn around, another faction declares war on me. As soon as I head in their direction a third faction declares war. before I can turn for a double-take, I'm at war with 4 factions! What's worse, is that now peace costs half a million at least, so I either die a slow death fighting or bleeding denars. It's maddening. What's the point if the AI decides to throw everything at me while i'm still, comparatively, the weakest kingdom? Adding insult to injury, for each of these factions I'M THE ONLY ONE THEY'RE AT WAR WITH. Devs, please fix this bonkers AI...
And for the record, I never execute in a futile attempt to avoid these exact scenarios (though I do keep them prisoner XP )