I've noticed on my Sweboz game that the Aedui and Arverni rout very quickly. I'm not using any scary units or anything, and they usually outnumber me. If I was fighting Romans and they had the same number of troops as some of these Gaul armies, I'd probably lose the battle. Usually about a minute after my main line engages, the gauls break and I chase them down.

I mean, I could easily make a joke about French people retreating, but that doesn't explain anything. Why do the gauls have such low morale?