Looks like a hawk attack to me. They hold them down with their claws on the side and then start to peck away. It looks just like how my hen looked on her back. We've learned, before we put up netting over our entire backyard, to check under the wings for wounds from the claws and not just what we could see on their backs.
Cover wound with Neosporin or something similar and then put a ouchless pad over that and I would wrap her all around with the ouchless tape to keep the bandages in place. They heal so fast which is a good thing.