From a evolutionary standpoint, it makes no sense to have a predatory animal that folds its facial skin back as a threat display. I could maybe see it as some sort of weird vulture-like scavenger thing. But hey, in D&D, evolution takes a back seat to the rule of cool - ask the insane wizards who came up with the bullette, owlbear, and other fan faves - and skull-faced critters are cool-lookin'.