I agree it's boredom. Best thing is turn them out, but SL is turned out, so I guess that's not a cure either.
Becky
Boredom can be temporary because she was tied. That's when Cyn does it. When she's bored or frustrated. She's gnawed at tie rails while we're on rest stops on rides... and she's chewed some scallops in her rails of the stall, if we aren't fast enough with breakfast.