Cribbing is a very different behavior from wood chewing. Cribbing damages wood (and other structures) from the horse bracing against it, but if the horses are actually chewing on it then you have a different problem.
There are a lot of theories behind wood chewing; not enough roughage and/or lacking in minerals is the most common one. The first thing I'd try would be making sure that the horse has access to hay 24/7 and take a close look at the diet to make sure it's complete. There's a supplement called Quitt that has a money back guarantee to stop wood chewing within 7 days; I would definitely look into that, too, if I had a wood chewer.
Boredom is another possible reason for the new behavior. It's harder to alleviate this one when a horse is on stall rest or has to be kept in due to weather, but there are some "toys" out there. I've had mixed luck with stall toys, but it could be worth looking into if more hay/diet doesn't fix it.
I honestly think they are eating it, but they get 5 flakes a day each of good hay and 4 quarts of high fat grain