The odd time Mountain Lions will take something bigger and leave the carcass right there. I came upon two baby Bison carcasses when I was out working, about thirteen years ago, and they were cat kills. They had rear neck injuries, and the guts were eaten, but little else, so guessing an older cat. No skull damage though, at first. Until the wolves, bears, and ravens found and scattered them later. The cat was working the edges of the herd.
I think trail cams are a great idea, to know for sure, but I suspect what you are looking at is varied kills, from various predators, where the skulls have been cracked open by a bear after the fact. They love that shit. Brains are fatty. Mmmm.
One time, my co-worker hid a wolf skull up a ways in the trees, under an old stump, where some asshat had killed a wolf beside the dirt road, and left the carcass lying there. He wanted to clean it off and mount it in his den.
The next day we stopped to pick it up, and the skull was missing. There were Grizzly tracks all over. He was choked. They'll find a carcass pretty damn quick.
Addendum: Looking at your pics, its not a fresh kill, so guessing a black bear got the skull cracked open, after the fact.