Also agree horse is bored and/or stressed. 

Personally, I'd consider it a red flag re: the experience of the BO that she thinks the chewing will lead to cribbing, and wants to put a cribbing collar on a horse that is NOT cribbing...

“It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so.”                                                                                                                           ~Mark Twain