Pistol butt can be caused by a tree on an unstable slope (how I hate that term) when the slope slides a little and the tree then goes to straighten up. Pistol butt is also caused when a log slides down against a young tree. The log will rot away, but the crooked stump will still be there. Really unstable ground won't have pistol butt trees, just leaners and down trees. The only leaning I see is caused by stuff other than light. Trees will grow branches on the side with the most light exposure so don't have to lean to the light. Thin a stand, and a few years later, the space will be closed up by the crowns of the trees left in the stand.