Up here in Oregon last year, there were issues with a local snowmobile club taking heavy earth moving equipment into national forest land and doing hundreds of thousands of dollars of damage by clearing their own trails. The few people that saw them just sorta assumed that they were supposed to be there tearing down all those trees. It happens...
The Forest Service here will tell you they simply don't have enough people to begin to patrol everything, and that they rely heavily on the "eyes and ears" of ordinary citizens to find out about a lot of the illegal activities taking place.
That said, clearing snowmobile trails using heavy equipment without permission is extraordinarily brazen.
perhaps a cow died and the rancher buried it? Or maybe it was some other animal that someone thought would be better off buried than left to rot?
A rancher burying a dead cow occurred to me, but dead cows are not uncommon and you see them lying around from time to time and, later, their scattered bones. I've never seen one buried before. And it's hard to imagine why trees would have been cut down and tossed in the hole if it were nothing more than a dead cow, etc.
Still, that, or something similar, strikes me as a best guess, at least so far.