That's terrifying! I feel badly because I was just thinking I wish it was this chicken instead of the other chicken that was killed last week. I think I know how it got in. It's a hoop house covered on the top and sides in chicken wire, HOWEVER, the chicken wire isn't zip tied very close around the pipe (tied every foot or so), so I bet it landed on the top and created a gap to fly right through :/
Also forget about the images of a hawk swooping down from outer space like a meteor and pouncing on you chickens. Hawks don't kill their prey like that at least not prey as large as an adult standard size hen or rooster. It requires quite a bit of killing to accomplish turning almost any brown egg layer into hawk bait.