Did it just start for no known reason? Have you changed her routine so that she is in the stall more? Have you changed what her diet is recently? Is her routine more traveling and showing than before, or is she suddenly not being used very much?
There are tons of reasons why horses begin to chew wood. It can be anything from dietary problems, boredom, stress, digestive issues, and even mimicking other horses.
I would look over her diet with a vet and see if there appears to be anything she is lacking. It could be as simple as a vitamin deficiency, or just needing more hay throughout the day. If she is stall most of the day, then I would recommend more turnout. If she is turned out by herself, I would recommend trying a pasture mate if possible.
Also don’t neglect to check for ulcers. A common cause of chewing/cribbing is attempting to relieve the pain of ulcers and other gastric problems.