It is hard to predict why your horse is behaving the way she is, Jessicahowever, my first thought would be boredom and frustration. Be sure she has access to hay overnight, and I would stall her with a few enrichment devices (you can find some fun lick and treat devices that can safely and easily be bolted to the stall). I suggest setting up a video camera and capturing her behavior before you try any interventions, and then during the interventions. You should be able to determine whether an increase in foraging opportunities decreases the behavior. Another option would be to put her into pasture with access to her stall or paddock, but I would make sure she can see and hear the other horses if she is left out.