I also had the same kind of problem on the battle map. it happened twice ...
First time I thought it was my keyboard but it happened again yesterday and plugging out my keyboard didn't solve the problem.

it's like I was pressing one of the arrow keys , moving the camera over the battlefield through the border... couldn't do anything except hearing my cav being killed by spearmen

The only thing I could do is change the camera to general view to finish the battle...

Very good thing in fact as I discovered this point of view that makes battles quite harder.