Even if horses have plenty to eat, they will still strip bark off trees. It is in part that they are bored... plus, a horse has a fairly simple digestive tract in that they normally, in the "wild" eat approx 18 to 20 hours a day. They do not "store" any of it like with an animal with a rumen.. so they are constantly looking for something to eat. If they get full, as often they will if given hay to eat continuously, they will get bored with the "same ole same ole" and go to chewing on other stuff. Have you ever seen the big rubber type balls that have the handle on them that they give to them to play with to alleviate the boredom??? Some are worse than others too.