I use to think it was because of a weakness that individual chicken was showing or an illness, but they are like children, all of a sudden they get mad at one of the group and they are relentless in their attacking. I recently had a Black Australorp, who was one of forty, raised with all of her sisters who became the victim. I had to remove her from the flock because she was being pecked at so much and was spending most of her day hiding.
She has now become the barn dog, following me around everywhere. After two weeks, I put her back with the rest of the girls and they left her alone, but she squaked and did not want to stay in the pen, so out she came again.
Yesterday, we had temperatures in the high 50's so I did some work in the garden, preparing for the day to plant the potatoes, and there she was following behind and scratching the soil where I was working.
If anyone can figure it out..........I would be curious!!!