This just happened and I can barely believe it. I was playing as Labrynna and I put Queen Ambi in charge of one of my armies that I sent to capture a rebel town. I also had recruited Ashei into this army. Since it's early in the game this army comprised about half my military force (those that were competent anyway), including units I currently can't replace. One turn later I find that both Queen Ambi and Ashei have defected to the city they were besieging and my army is gone. Neither of them had loyalty issues, and Ambi even had a decent chivalry score. Was this just a random fluke or is it a bug? I didn't even think your faction leader could go rogue.