It's on the The two Towers DVD, the 4 disc set, explains it all. It's really nothing to do with being stupid or smart. The characters weren't running away from the battle as such. It involved characters at the back of the battles, the farthest distance from the enemy. Any characters that were initially facing away from the enemy would run in thier facing direction, being so far from the enemy so as not to "sense" them and therefore wouldn't change direction to confront them.