I've also experienced the AI immediatly withdrawing his general. It was a unit of urban militia. He fielded a small low quality army with quite a lot of siege engines. I was aided by my allie the Germans (I was the English).
I found it a sensible decision as the AI army (rebels or French) was inferior in quality and quantity. It had no hope of winning the battle so it saved what was woth to be saved ( in its opinion). I wasn't able to catch up with the general as my speedy cavalry first dealt with the artilery.