its the crappiest of glitches basically it sin native warband.

What happens to the player is you are seiging and the defenders rush out to attack you, that scene always has like 40:1 enemies to players. Most likely som of your army is wounded and so the ai clickd the sally forth button basically. You get launched in with like 10% of you rarmy and they get like 50%

basically never attack without dcent numbers 150+ to avoid that