My new horse does this and just due to lack of an empty ring at the times I normally ride I haven't been able to do the training with him. While he stands perfectly still if the mounting block is in a corner, he will do the same thing, swing his back end away or step forward when the mounting block is in the center of the ring.
Laura mentioned Clinton Anderson's methods. Rather than the sending method, I use the hindquarters yield for mounting block issues. Essentially, when the horse moves, make her move. I hold the reins, step back to the flank and make the horse spin a few times. I stop them, give them a pat and move back to the block and try to mount. If the horse moves again, I repeat. Eventually the horse decides it is much easier and nicer to hold still then to go into those spins or be forced to work. Before doing this just make sure you have protective boots on just as a precaution. The spins don't have to be particualrly fast...just make her move her feet.
Another similar method might be to have a lunge line attached. If the horse moves, lunge at a good strong trot, go back to the block and try again..repeat as needed. The only issue there is that when you do finally mount you would have to figure out how to unsnap the lunge line :)
The last horse I had was a proverbial terror at the block and it took me 15 mins of the hindquarters yield exercise before he would finally stand still. He never moved off again and I could mount with a loose rein.
As an explanation for the sending exercise, it is sort of like a lunge on a quarter circle (or a straight line). Rather than have the horse circle around you, you "lunge" in one direction until the hindquarters pass you then stop the horse. Change directions and lunge the horse in the other direction, again, stopping them just after the hindquarters pass. The horse should never get to your side or behind you when doing this exercise. I used to have fun with this one with my last horse, doing this same exercise wihtout stopping with him going back and forth in front of me while I was walking from one end of the ring to the other.