This is from my vet book Horse Owner's Veterinary Handbook by James M. Giffin, M.D. and Tome Gore, D.V.M. published in 1989:
"The causes of wood chewing and cribbing are not known. Observation suggests that one or more of the following may be causative or contributing factors: social isolation and lack of companionship; boredom; frustration; lack of opportunity to release tension and nervous energies; need for more fiber; imitative behavior; and vestigial browsing behavior. An hereditary predispostition for cribbing has been identified in some Thoroughbred families."
"You know, for as long as I can remember, I've had memories." ~Colin Mochrie