Thanks for the information. It's only a guess but I think she was flock master but when the others matured enough she was knocked down in the pecking order. It is possibly one or some of the pullets but I think it is mainly the cockerel. The ages are about right.
I had a flock with some mature hens, one of them the dominant hen. I also has a cockerel and some pullets growing up with the flock. When that cockerel matured enough he stood up to the dominant hen and defeated her. For two days he would not let her near the rest of the flock. The other hens and pullets were not involved. It was pretty vicious for a couple of days but no one was injured so I left them alone to work it out. After a couple of days they did and became best buddies. But I did watch closely. They were fighting, one of them could have been easily injured.
Not every scenario is exactly the same. Sometimes chickens get injured. Sometimes this changeover is so peaceful you don't even notice it happening. Most of the times I have a cockerel take over I don't see anything going on until one day I notice he is in charge.
Since you say some pullets are involved I think it is possible that not only is he taking over the dominant flockmaster role one or more pullets are trying to take over the dominant hen role. They are adolescents becoming adults. There are growing pains. Keep an eye on them to take action if anyone is injured, but otherwise I'd let them work it out.
Good luck!
