I very briefly had a horse who was a cribber. Cribbing is when the horse grabs something with his upper teeth, arches his neck, and gulps air. They usually grunt while they do this. It was the weirdest thing. His eyes would roll back a bit too, and I was reminded of a drug addict getting high.
He had a collar, which never worked. We tried treating the wood with various substances, which never worked. Some say its an anxious habit horses pick up. Some even say its due to a traumatic weaning experience. All I know is it seems nearly impossible to stop. The only thing that seemed to help him was constant turn out and to keep him away from solid wood/metal objects he could grab onto.
Some suggestions to reduce cribbing :
Provide free choice hay/forage. A wild horse is grazing all day. Stalled or penned domestic horses may develop cribbing due to boredom or stress from not being able to act naturally.
Allow the horse plenty of turn out time. Many cribbers who display this behavior in a stall or enclosure will not crib in a large field.
Provide companionship. A lone horse, or a horse kept in stall with minimum contact with surrounding horses may be bored or stressed.
Vets say that cribbing releases endorphins, so horses get in the habit because it is like a drug, and they become addicted. There are drugs you can give a horse that blocks the release of endorphins. Therefore, the horse will discontinue feelings a "rush" when they crib, and may stop.
Some horses will never quit cribbing, and all you can do is manage the horse so that its teeth and digestive system are protected as best they can be against the affects of cribbing. (Regular dentistry, balanced diet, etc)