I have had this same issue, ALTHOUGH it is not every time I go into random spot on the world map, it happens about 30-40% of the time. I am playing as Humans, and have had this problem in all 3 regions humans control. When a large group of roaming neutral units walk towards my fortifications, I move my heroes, range units and knights all towards them, the game lags badly and then get a popup saying the game has crashed. It has been doing this for the past week or so, although I had not played a lot in the few months prior to that.
I have played a few sieges against the NPC towns and it hasnt happened there, nor has it happened in a PvP battle or quest.