Share 'HORSES EATING BARK OFF TREES'

Anyone else having this problem this year?  Haven't had any problems with this in a few years.  I have the same horse I had last year and he had never bothered any of the trees.  The horses in the pasture next door are doing the same thing.  Our horses have a salt block, a mineral block, hay available at all times and get fed good twice a day.  Any ideas?