Horses often chew or crib when they are bored.
As stated by others more turnout or plenty of hay can keep a horse busy.
Adding a hanging toy or something your horse can play with may also help.
If nothing above works try looking into nutrition deficiencies, and possible stress.
Also check out this article http://info.millbrooktack.com/bid/142587/Cribbing-vs-Chewing-and-5-Ways-to-Get-Your-Horse-to-Stop
It might help you a bit.