You know I don’t know much about horses but what I do know is that God created their mouths, in particular, to constantly “pull and chew”, “pull and chew”, “pull and chew”, about all day long. Horses are continuously working their mouths.
When they can’t, due to being stalled or being away from grazing for however long (dependent on the horse), an anxiety will build in their mouth and they will satisfy it by chewing the stall, cribbing, or grinding their teeth.
Solve the problem by finding a solution to keep your horse’s mouth anxiety down. Slow feed hay nets are a good choice for some as well as free-choice mineral blocks (the big ones that go on the floor). My OTTB has found his mouth satisfaction with alfalfa cubes for they give his mouth a good workout when he tries to break them up and that workout keeps the anxiety down for quite awhile afterwards. Plus, since I put the alfalfa cubes on the stall mat that keeps his head down which further aids in calming him. I hope something I mention give you an idea to try.
Too bad horses can’t chew bubblegum.
Do check with your vet for any health problems that might be causing it if keeping her mouth occupied isn’t satisfying her.