I had a similar experience. I was walking with my main hero (but with a small army) past an enemy town with a hero and an army. I saw that his army was about 3 times larger. I thought I needed to get out of there quickly. The AI was in the castle. I took all his mines. The AI just sat there. I ran around his territory back and forth, collecting resources. It just sat there. I got tired of it and went to another part of the map. The AI just sat there and didn't even try to take back the mines. I wondered for a long time, is it embarrassing for him to take back what's his after someone else? :)