It seems strange that an animal would chew the head off. I’ll admit I’m not an expert.
It may not have had much holding it on, at the time. The shoulder blades would be easy to drag off, too, unless clothing made it difficult. I used to hunt antlers. If I found a shoulder blade, I’d start circling around. There would often be an unrecovered kill from a hunter nearby. There’s little holding a shoulder on a body that is decaying.
It's not unknown. Among mountain lions, for one. After a big cat has gone for your jugular, there's not a lot holding your head on, especially if you're small-framed or a child.