ShiviBear
Songster
I think it's because they feel she is weaker because of the injury and her weakness poses a threat to the flock as a whole.My chicken was bitten badly by a dog. I keep would clean, cut feathers back and kept her inside in a wired dog cage. She did heal but the other chickens would attack her when reunited. Not sure why.
My girl Vulture who was attacked by a hawk, had to be separated with a few mild manner hens for at least 3-4 months after her injury healed. She was one of the top hens before her injury, now she on the bottom rung, even though she is healthy and a prolific layer.
Chickens will actually kill other sick/injured chickens to protect the flock (I've read)