Why do horses chew their stalls?

( I apologize for missing a few days. I didn't have time to queue posts before leaving for a convention).

Sometimes if you look at a stable, you'll see clear evidence that a horse has been chewing the wood of their door, wall, manger, etc. Why do horses do this?

One reason is boredom. The other reason is not getting enough fiber in their diet - horses need a lot of fiber. Sometimes a horse will get in the habit of chewing wood and never break it. This is often seen with horses that have been malnourished in the past. A change of diet can solve the problem, but sometimes the only solution is to cover all the exposed wood surfaces the horse can chew on with rubber or metal.