Is that the (sole) reason? More than one martial arts trainer has told me that if you are definitely about to be involved in a fight -- and you can't run away

Is that the (sole) reason? More than one martial arts trainer has told me that if you are definitely about to be involved in a fight -- and you can't run away (number one preference) -- if you have a full bladder you should consciously empty it. Wet trousers are far preferable to having your bladder rupture, so it's a sensible pragmatic (or instinctive) action. I'd imagine there'd be similar benefits in evacuating your bowels.