hm... do you keep your army in formation? Anything different from any regular battles?
I know that before the patches if you had a large army the back of your army would drown in the lake behind where you spawn (against traders there are only 2 battlefield maps). They didn't die but they all get wounded after battle. So if you have large battle sizes, about 200 of your men would drown at spawn. I think this was fixed with the current beta patch.