IMO- This is how it went down: The hawk, being a young hawk, is not yet a skilled hunter and doesn't know that he has to catch and kill his prey early enough that he has time to eat it. So, he catches and kills the chicken and starts eating it, starting with the head as hawks do. They start with the head because it's the easiest first few bites, and they eat everything but the entrails (guts), even the legs and feet. Well, he's eating his dinner and being distracted by the other chickens as it starts getting dark. The other chickens start heading to the roosts, whether it be in the coop or in the tree. The hawk makes a move for the RIR as she's heading into the coop, and almost gets her by her leg. She gets away and the hawk finds himself inside the coop and it freaks him out so he tries to fly out, but there's no way out except through the door that he came in. He doesn't remember how he got in because he was persuing the RIR at the time. So, by then it's completely night and he decides he'll figure it out in the morning. And the rest is history!
Just my opinion.
Get some goats or some dark colored turkeys to deterr any more hawks.