Jeff
I think it is because in a good downswing pivot, the right shoulder moves "down, out, and forward" while the left shoulder moves "up, in, and back" (or something like that). With a steeper right shoulder (or higher, however you want to look at it) the more the shoulder has to move
down with less
out than versus a flatter (or lower, deeper, however you want to think of it) right shoulder. A slicer's pivot has too much
out coupled with not enough
down with the right shoulder. It seems like a flatter shoulder turn would help alleviate some of this, while a steeper shoulder turn would only worsen the move.
Again, I'm not trained or educated like Brian, Jim, etc. This is how it seems to make sense to me however. I hope Brian or Jim can chime in here to shed some light
.