When it first happened I put it down to chance. But I saw it three times since. I sometimes like to turtle and see if I can handle what the AI comes throwing at me. So I started a small map with two kingdom empires and played Umber. I found a nice big valley I could turtle up in and built my 5 cities in there. Soon the two kingdom AIs were at my doorstep and predictably they declared war soon. SO far so good. Since then all I have been doing is to raze some of their outposts and kill off the armies they keep sending into my land and now they are suddendly ready to surrender?? I do not even know where their cities are. Not to mention they got 3 more cities than me admittedly smaller but still. I think the AI relies to much on military power to weigh such decisions. And since it threw all it had at me there was so army left so..... Yay surrender!
PS.: I can provide a savegame of this particular instance if needed.
P.P.S.: It sees especially puzzling that they would comtemplate surrender but not a simple peace treaty.