aww come on, it's totally possible, once you think about it.
the catapults are simultaneously full of adrenaline (think incredible hulk here) contemplating the task ahead of them, and completely and utterly scared because cavalry is like, i dunno, same as santa claus, they thought it was only a fairy tale told to scare children. no wait, santa isn't to scare children, scratch that and insert "bogieman". then again, their commander wouldn't tell them to try if they had no chance (they're naive) and so they try the theoretically impossible and succeed (think of dumbo and the magic feather here). so, sometimes, the combination of those factors ends in a withdrawal. naturally none of the other catapults believe their story. they see the remaining health on the one that did withdraw and roll their eyes thinking "whatever caused that the near-death experience gave him visions". and so the whole santa claus/bogieman/incredible hulk/dumbo cycle continues, game after game, for each new generation of catapults.