This is a very common problem especially with young horses. My old horse used to do this and we had to pull the cross tie to the side. One thing that could be causing this could be the fact that she’s bored in her stall. What can help is a hanging treat or a jolly ball this relives anything that could be causing this. You could also turn her out and this could help her. Just keep this horse busy so she takes her teethy behavior out and she doesn’t bite things like cross ties.