It is because of the courseness of the winter coat. I have a similar problem only mine is where my leg contacts my horse (and he does NOT need a lot of leg, my leg is stable etc).
The only way I've found to fix it is to clip the horse.
Originally Posted by goingnowhere1
my horse has the exact same problem. And the saddle was checked and it fits perfect I also use a raiser pad and sheepskin. I don't really have any advice (still trying to find out the problem myself) but I'll be happy to see what everyone else says
If you need a riser pad then your saddle does NOT fit.
If your saddle fits without it and you put a riser pad in you will cause the saddle to sit back and it will no longer fit.