Cribbing is the behavior in which a horse sets his teeth on an object and sucks in air. It should not be confused with wood chewing, and without being able to ask additional questions as to your situation, it's a bit difficult to respond.

There are likely several reasons why horses may start to crib. The two most prevalent theories are that it is caused by boredom or by pain or discomfort of some type, which could include gastric distress or ulcers. The only way to know if he has ulcers for certain would be to have him assessed by a veterinarian.

The diet you are feeding sounds fairly high in energy. It could be that the horse is taking in more energy than he needs based on his daily workload or turnout schedule and hence has started to crib as a result. If he does not have access to daily turnout or is not exercised regularly, that might help. It may also help to switch to oats (if the horse is mature) or something a bit easier on the GI tract to see if there is any change in behavior.