Some of the predators that normally go for the head first are hawks, owls, weasels, bobcat, skunks, and raccoons. There are probably others. Just because a predator normally does something doesn’t mean all of that type of predators do the same thing. Maybe they get interrupted, maybe you just get a strange one. Still tendencies are about all you have to work with.
I’ve had a chicken killed by another. A cockerel killed a rival by pecking its head. The head was not missing but the back of the brain was opened up. Not a pretty sight. The only time I’ve seen a head totally missing was when an owl got one. It just ripped the head off.
Did you examine the body of the dead chicken for wounds. Often you will see claw marks, like from a bobcat, hawk, or owl.
I tend to think weasel too but boy these things can be tough to figure out.