My first guess as to why would be boredom. I've also noticed when there is a weather change (Hasn't it been raining there alot?) that horses will suddenly start doing strange things, eating the barn down, eating the bark off trees, etc. I've had horses that didn't ever do things like that suddenly start, so it may not be anything serious.
Becky
Yes - weather change, lots of rain and mud, and we started putting them in at night.
And Cyn did chew a spot in her stall... just a small spot, and we put a metal plate over it and she hasn't done it again, yet. But she'd never done that before. I really hope she stops, I'm not looking forward to having to paint all the wood in the barn with that black stuff.