hen's ganging up on one of their own- seasonal pattern?

I'm glad you posted this, I was just looking through to post this today, but was visiting about egg laying behaviors first! I've had this SAME problem twice now, and its happening again! It started when the weather turned much warmer. ALL my birds were raised together, they have tons of room in the coop and yard. Then suddenly they seemed to single out one hen, who actually was NOT low in rank and someone picked a fight with her. Once that happened they all began to pick on her. She got so depressed she stopped eating, the pecking kept happening, they pushed her out of food when she would come around. I would have to feed her separate. But she eventually stopped eating and not coming out of the coop at all and I found her dead one morning. They had pecked one of her eyes in the first fight I was doctoring and she actually lost it, it seemed like a sign that they all just picked up on.
THEN I had gotten 5 new birds, they were bantams (I had all standard birds except for 2 bantams that I loved which is why I wanted a few more) and they did the same thing to one of them, they pecked her eye and she almost lost it. I caught it in time to save it, but they soon killed her and I found her dead in the morning. Thing is, I had them separated where the bantams could get from the coop to a separate run in the day that the big birds couldnt get too and for weeks there were no problems. They killed her either at night or early morning. NOW my poor rooster had a hen attack him and again...pecked his poor eye!!!! I'm at a loss! He has begun to lay around just a bit more and I'm wondering if I need to separate him NOW before he gets depressed. But let me tell you, he is NOT happy at all and I dont want to make him mean being away from the flock. I just dont know what to do at this point. I hope this reply bumps your post up and someone answers!!!!