Is she Arab?
I have owned many many Arabs who do this for fun. It is like thier head is not really connected to anythign and WHOOP up it goes, straight up and to heights that should not be possible for an animal that size. They go galloping about and up goes that head for the sheet joy of running.
Of course, they will do it under saddle too if not taught otherwise. As a kid a had my nose broken twice (the cartlidge has never healed right) and once put my front teeth all the way through my bottom lip resulting in the need for three stitches from my mare tossing her head when we would be out and about on trail.
Where I am all for making sure there is no pain, it might just be joy of life too.