I don’t usually think of the Great Horned Owl as being vulnerable to predation, though they certainly are, as is evidenced by the photographs below. Its not difficult to imagine a Bobcat getting the drop on one of these birds if the owl was momentarily distracted. Maybe The bird and the cat were going after the same nocturnal rodent, and the Bobcat opted for a meal upgrade.
I asked around about this situation, and the folks who know much more about these things than I do tell me that the intact nature of the ribcage combined with the widely scattered remains suggests that the carcass was likely picked clean by vultures. In all likelihood the owl was injured or sick and expired on this hillside of natural causes long before the vultures found him. Wildlife CSI!