Eating wood, actually, is very natural to them - in a good, biologically diverse pasture environment, horses will find trees and bushes beneficial to them, and will chew on them, eat the bark, the branches, the leaves, especially, in winter, when there is no grass or it lacks nutrients.
Also, when we tie a horse, we have to keep in mind that even we don't tie them for long by our evaluation, it might seem long to a horse with a very active mind - each is individual. ;) My gelding, for example, gets bored within a few minutes of being tied - he behaves well and does not make a scene out of it, but you can tell. Then again, he's a type who needs constant stimulus to be happy!
I have come a long way, to surrender my shadow to the shadow of my horse.