ORIGINAL: HuntingEd
Most of the bald spots I've seen come from going under a fence, I guess as they get older they dont mind rubbing against the fence...
Either that or it could be scars from fighting that just prevent hair from growing back...
This would also be my guess, I have seen and killed quite a few bucks that have had bald spots on their back, and there are alot of old fences so I guess it's from them crossing under them.