Bison Horn 'Sharpening' Tree?

I stumbled across this tree in Badlands National Park, South Dakota. I was going up White Butte in October, 2013. The area was full of bison, including a herd of about 60. This spot was out of the way (I was trying to avoid too many muddy crossings of Sage Creek) but full of signs of bison activity. I am wondering if this tree was/is serving as a place where bison 'sharpen' their horns, or at least rub them for reasons unknown to me. Has anyone come across this kind of thing, or is there some other explanation entirely?