It depends on how the enemy army is grouped, I think. I *think* it may be a question of "rearmost" in terms of squads.
If they are all lumped together, the troops *will* go to the rearmost.
I made a little post here with an anecdote which may illustrate what I mean:
http://www.shrapnelgames.com/cgi-bin...&f=74&t=002246