OOOOOh, this can get to be a big problem. Try doing a search here, there was lots of discussion about it recently since many people were having the same problem.
I had to remove our little Red Sex Link because her lower back was picked completely bare and starting to bleed. She's been inside in a dog crate for a week.
My theory, after doing much on-line research, is that its a combination of winter boredom (they girls don't get our of the run into the yard much, and when they do there's only snow to peck at) and a protein/amino acid deficiency. I figure that the 16% layer feed is supplemented in summer by all the bugs they are eating. Now, it's all they're getting. Plus, when I bring them treats (by which I mean healthy, organic scraps of vegs, grains, meats) they go absolutely bonkers for the protein, and then eat the rest later.
So, I am now supplementing with:
-black oil sunflower seeds, for protein
-meat scraps for amino acids and protein (organic meat/fat table scraps, canned tuna, etc)
-plain organic yogurt
-Avia Charge 2000 in the water daily , for amino acids
I have not seen any pecking among the other 5 girls since I started this, but the "victim" hasn't been re-introduced to the flock yet.
Here's a good article:
http://www.organic-vet.reading.ac.uk/Poultryweb/disease/feath/feath.htm
good luck!
Stacey