Last year, I spent a week with Mike Rogan (if you don't know of him, he's been on the PSIA National D-Team for quite a few years, and is one of the guys who leads PSIA's direction). At this event, we spent a day talking about stems. We came to the conclusion that there were 3 possible causes of an unintentional stem.
1) Rotating the downhill leg for a variety of reasons, such as speed control (a "check") or looking for rebound out of the ski, or simply a week inside half, not finishing the turn with the inside ski
2) Over rotating the torso
3) Over countering the torso
So, until there is a visual observation, it's hard to correct the issue because you could go the wrong way and actually make it worse