The thing that bothers me is that these AI "surrenders" are random, often when they are not really threatened or in any danger. AI surrenders make sense when they are about to be crushed. Japan offered everything to me, and that's good AI. He had a single archer and a single city, I had 2 warriers, a horse archer, and a horsemen.
But Russia? With her 3 cities, and even military, I am 100% certain I could not have taken a single one of her cities even if my units teleported next to her empire. Why am I so certain? Well, With my above mentioned army, I barely won with a 20% health horsemen, having lost both warriers, against Japan, the only civ NOT in the classical era, and who had a single archer.
In these stories that I'm reading, it seems similiar to mine. A small change, military ratio goes from 50/50 to 52/48 in your favor, and the AI panicks and gives the human player everything.
In my case, the only thing that I could think of that could cause this is that a horsemen popped from one of cities. Now that I had TWO horsemen, I dunno, it may have pushed the military advisor's assessment from even to slightly in my favor. Certainly, Russia was in no actual danger from me, before or after my second horsemen. In between us lay Greece, which even with my two horsemen is "wayyy more powerful" than me according to my military advisor.
This is definitely a bug, and it is basically the AI commiting suicide.