Yes, backing up, rollbacks, and riding in hills helps to build the muscles in their hind end. However, she can have the biggest hindquarters in the world, but if she does not engage them they are useless.
Can you post a video or at least photos of running the barrels? It is hard to tell you why she may be turning on her front end without seeing her run. It could be the way you are cueing her or the way you are riding.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.