We have seven almost 12 week old pullets. Two of them are Welsummers. Just recently (last week) one of the Welsummers started picking on the other. The bully is not at the top of the pecking order (of all the girls) from what I can tell, two of the Ameraucanas sort of share that spot

She doesn't pick on anyone else and nobody else picks on the one she is picking on. She grabs the feathers at the top of her head and pulls them out. I don't worry about it too much while they free range, but she is secluding her from the others and not letting her sleep in the coop half the time.
Any ideas on what to do or why she is doing this?
Also, my Welsummers are the opnly ones that do this so I thought I'd see if anyone else's do this. They puff up all the feathers on their head (like a dog's hair sticking up on the back of it's neck) when they are trying to look all big.