Are you mounting from a block or from the ground? If you are mounting on a block does she move from the block when you stand up on it, or when you put your foot in the sturrip, or will she not go near the block at all? If you are getting on from the ground, is someone holding the sturrip on the otherside to reduce saddle twist?
Isolate at which specific point she is moving. It could be that she has learnt to associate the foot going in the sturrip with the saddle twisting across her back, which causes discomfort. If you are getting on from the ground try using a mounting block, or try getting on without using the sturrips- ie vaulting from the ground/block. See if she improves. It could be she is in pain from the mounting process.
If you think she is just being a pain in the butt because she is testing you or something:
I would get a designated 'mount up' area within another area where you are relatively enclosed but its still big enough for you do do ground work in- eg a corner of the yard. Place her in your mount up area calmly and proceed to get on her. As soon as she takes a step away from you get after her butt end and make her circle around you. Don't be angry, just purpeousful in your body launguage. Let up when she is responding to you (and preferably standing facing the direction you want her to be facing for mounting), change your body language to 'relaxed' mode and calmly place her back in the mount up area. You have to make this area calm and relaxing for her to stand nicely in. Make it her rest area, so she learns standing here and letting someone mount is less effort then moving out of the area and getting worked.
She should learn fast enough that stepping away= pressure and circles, and standing makes life easier.
We lose ourselves in the things we love, we find ourselves there too ~Kristen Martz