The buck I killed last year was bare skin in about an 8" diameter right at the top of his back right above the front shoulder blades. The taxidermist told me it was from them scratching ticks off their back. Laying on their back and rolling around. He said mine had ticks all over right there, as well as many other ones he sees.
Made sense to me, as that's just about the only spot they can't reach. That's the first one I killed with a spot like that and I assumed it was from fighting and getting his butt whipped. He had scars all over him. I guess it could be one of many reasons