I experienced this a few times in my early days of playing MTW. I soon came to the conclusion that it was my lame tactics that were causing the rout. And as Selbourne states it makes sense that men should run after being given suicidal orders. Put yourself in the baits shoes, they have no idea of their generals master plan, all they know is they are riding around under the nose of the enemy back and forth getting tired and so who can blame them for running?
The solution is to develop better battle tactics, don't rely on silly drawing out tactics when you can learn to defeat a well positioned enemy (much more satisfying also).