Okay, here's what happened, as I saw it. You might draw a different conclusion. I have six hens and three pullets. I also have two dogs, a schnoodle and a yorkie. My schnoodle, Greta Garbo, is 12 lbs and 12 yrs old, but would easily weigh 20 lbs if she had her way. The yorkie is a 5 lb 10 yr old named Rudy Valentino. He's small, but has a really strong prey drive. Greta has, on a couple of occasions, gotten into the chicken yard. She ignores the chickens and heads straight for something to eat. Her strategy is, first whatever food scraps are in the yard, second the chicken pellets, and third the chicken poop. She paroles the perimeter of the chicken yard anytime she's outside for all the same "treats", sticking her nose under the fence as far as can. So, this morning, 6 AM, before coffee mind you, we were all outside. Me doing my chores, the dogs and chickens doing theirs. I had finished cleaning out the coop and was getting ready to clean up after the dogs. I know, lovely way to start the day, again, before coffee. So, I hear a ruckus at the farthest end of the chicken yard. Feathers flying, squawking galore, all the hens ganged up together. I thought they had trapped one of the pullets and were ganging up on her. I start yelling and running up the hill to the rescue. No small feat for an old lady. Then I hear, along with the squawking, my dog yelping. By the time I reached them, Greta and Rudy are pulling away from the fence on one side and all the hens are dispersing on the other side. The pullets are hiding under a bush, far from the fray. No visible injuries to anyone. My best guess is that Greta was attempting to either reach a "treat" of some sort, or get entirely under the fence to get to all the "treats" and, I don't know, maybe got her head stuck and one of the hens started pecking her, then the rest joined in??? Rudy had either come to Greta's rescue or was hoping to get in on the action and get access to the hens. He will kill a hen in a heartbeat, even though he's nearly toothless. I've seen the hens peck at Greta before when she sticks her nose where it doesn't belong, but I've never seen them all gang up on her. After I came back in the house, I did see all the hens gathered together, maybe plotting their next fight...or congratulating each other on a fight well fought. Whatever happened, I hope Greta learned her lesson. If it had been Rudy, he'd be looking for revenge. Oh, and I finally got my coffee. 
The chickens have seen Greta stick her head under the fence almost on a daily basis. And only if she's after food they are also interested in do they even bother to peck at her, missing her more often than not. I think she must have gotten caught in the fence and was maybe struggling. I have seen hens pile up against a chick or a young pullet. And I had a hen who was so mean that I had to rehome her. She had a bff who would join her in an attack. I kept her, but the other hens were not so forgiving. Took them a good 6 months to forgive and forget. I would just love to know what actually provoked the whole thing. 
