Lots of good possibilities here:
_ cinching to quickly
_ cinching her to tight at once
_ no respect
_ back problems
_ saddle fitting ...
I would look at all these possibilities one by one and rule them out one by one.
Does she just do it when you are preparing/riding her or does she do it with other people as well. If she just does it with you then I think it would have something to do with respect plus the fact that you might of just been a soft touch with her and she is taking full advantage of it.