Had this ctd just now after starting a brand new game. I'm not sure exactly what caused it. I had the majority of my starting army selected, I believe 4 legions. I gave them the order to move north so I could scout out the next mission just north of Lugdunum.

Here is the crash dump.
http://www.filedropper.com/hegemonyrome1999rev30687may09431

I get that whenever I have 4 or more units selected and order them to move in such a way that it attempts to create a formation, is that what you are doing?

That sounds exactly like what I did. Although I had been moving 5 units before when you count the cavalry unit. It only crashed when I used 4. Never seen it before now.

Hm maybe it is just 4 then, after the first crash and then 2 more to recreate it I didn't think to try with 5.