I'm pretty sure it's mostly from fighting, I went back through a couple dozen picks of big bucks working scrapes last night after I posted. Most of the big bucks racks are clearly off of there backs while using the licking branches but I'm sure it depends on the level of the branch's.
As for fences, The bare spots are mostly on the sides of there shoulders and some times all the way down to there brisket. Sometimes I have seen it bare on top also but the sides are bare too.
Last year I was able to watch 2 big bucks in a all out fight up close and let me tell you it was vicious, There was hair on all the walnut trees around the fight area and it looked like a rototiller had gone through when they were done.
What I noticed was that there heads were down almost on the ground most of the time and that there racks were pointing up and back. Even if there racks weren't touching there shoulders the other bucks tines were at times, I think maybe that's part of it.
Eric, Years ago at this same place the land owners dog would come through and roll in some of the scrapes and then pee in em but I have never seen a buck do that, Pretty cool. Wasn't Ken's dogs either. :D
Tracy