Ah, I was waiting for the first report of this! Devin's Cavalry brigade is supposed to act that way. This was setup this way to simulate how it was in the real battle. Devin's regiments were not heavily involved in the engagement, so they are set to want to retreat very easily. It adds to the challenge. If you really want them to stay and fight, you have to TC them.Might post it here since it's the same scenario.
The cavarly doesn't want to stay dismounted and seem to think that running away is a good idea.
And it crashed a few minutes in when I was constantly trying to get them off their saddles.

About the crash, see if you can reproduce.. If it happens every time, report back.