Every winter, this seems to happen to my mare...she is a very "wiggly butt" mover, IE she has a naturally back and forth movement rather than up and down like some other horses. What seems to happen is that with the back and forth movement, the saddle pad moves and rubs off the hair underneath where it's moving. The attached pics are of my mare's back and you can see where the pad has rubbed on top (hair is really thin and appears to be dark since she has black skin and you can see it through), and you can tell where down the sides of the back of the pad it has broken the hair as well.
The strange thing is that this rubbing is BEHIND the saddle, NOT under the panels directly (we placed the saddle on her back in the correct spot to make sure of that). I have tried using a really thick poly pad, a super thin baby pad, and everything in between with no luck! I've even tried taking some of that "sticky" material that you use to line your cupboards with and putting it under the pad and it just moves right along with the pad!!
Anyone else have this issue and how did you correct for it? For now, when I'm jumping I just use a fitted pad so that there IS no pad back there to rub, but what the heck will I do for dressage??