I have had exactly the same problem, and probably for the same reason. In my case I've discovered two issues. First, while my toeside turns are nice wide even turns, my heelsides are much sharper -- basically more of a braking maneuver. When I've forced myself to open up the heelside turn, I get that "fear of speed" feeling, so there's obviously some mental issues there.
The second problem is that I've not been shifting my weight back over the course of the turn. With my weight always forward, of course the back end can break free.
If you can do some turns on some relatively untouched snow and then look back at your tracks, you'll be able to see if you're having the same problem.