Horse Saddling Question?

This is a weird occurrence that has happened a handful of times. What happens is when I do up my saddles cinch (western) sometimes on the loosest it can go, sometimes when I am tightening it, my horse leans back so her front legs are planted but she leans back like the opposite of when they get up. I thought it was just a weird coincidence but it happens more and more (but still only once even few months). She did something similar the other day when I was tightening it in the arena. She went down in the front on the front of her fetlocks kind of. After this happens I have to loosen the saddle and get her back up which I then reposition it and tighten it again. The thing is it was never out of place before (to my knowledge).