Maybe it was because they fought next to a unit of raw swordsmen?
Or that they were just generally in a bad mood over having to march to war with press-ganged teenagers with swords.
It has NEVER happened that Bjørn has made a mistake! (And by never I mean it happens
)
Actually your veterans propably held their ground as well as the rest of the troops. You can expect that your raw swordsmen and your crossbows ran away under hostile fire. That your veterans wound up exhausted is actually just a definition of a wound level.
But the general impression was the Ilienese troops ran away. Which is why you should have spent money on a unit of spindoctors. I'm afraid Bjørn is the equivalent of "your spindoctor from hell", he will screw you over in places you didn't know you could FEEL sore...