Have you seen the deer rubbing against the trees. I would have thought the deer might be eating strips of bark.
Are there pin holes in the trunk of the tree anywhere indicating a beetle/borer infestation?
You could take some pictures and go to your local county extension office for some help.
You don't say how old the trees are or if they have been staked. If they are a few years old and have been staked you may want to try taking the stakes off and letting the trees just sway in the wind. It's been scientifically that doing this allows for the trunk to get stronger then if it remains staked.