"I'm guessin some kind of skin disorder or problem and the deer has probably chewed that area. I've seen similar bare patches on captive deer"

Hard to say... I cant really tell in the pic if thats just hair missing or hide & all. One i spoke of you could actually see the red muscle of the deers back. But that was early in the year and a very young deer with little or no fat where the tear was, so you could easily tell you were looking at raw meat!