One other REMOTE possibility is lightning. I realize that is a stretch but its not unheard of. The fact that it was a buck with antlers and was with two does, makes me assume it had to happen in late Sept-Oct when lightning is not as common, but in my area, lightning can occur any month. If this is the case it could be possible a tree that was struck can still be identified, maybe not I've seen lightning do some strange things. Livestock, especially cattle get struck and killed fairly often, I have first hand experience with this. Those deer could have been bedded there, got struck , and the critters feeding on the carcasses could have created the opening. Just my guess, I would not rule out Mountain lion either.