If there is nothing bothering her, it is most likely that she wants to avoid work.
A horse I ride does that every time I get on... he swings his hindquarters the other way and/or moves forward.
What I do, is I keep the outside rein shorter than the one closest to you, as I walk up the mounting block, he turns or moves forward, so the second he does it, I hop down and make him back up to the right spot, no matter how long it taked, never lead the horse back around to the mounting block, as this will continue for hours, make sure you keep backing and turning till in the same spot the horse moved from.. its alot harder to have to back and turn for a long ways... he will eventually learn not to do that...
Another way, would be to stand the horse next to a panel or a fence, bring the mounting block over, make sure the horse's right side is right up near the fence(enough room so can't swing)... gather up your reins, shorter on the right side, and climb on... making sure you keep your legs and reins steady when getting on so doesnt move forwards or backwards,
If the horse backs up while trying to mount, try putting him in a corner of an arena or fence, so that his side and bum is blocked in :)
Hope I helped..