Chickens can be the most mean, cruel and brutal animals that you will ever see. They will mercilessly weed out the weak in a hurry. If conditions are not right they will completely eat each other if you don't intervene. Just like many have told you there are many things that will cause this. Most likely they are too crowded and are bored. They need lots of room to sort themselves out. There are many things though that will cause this. Too hot, Too crowded, Vitamin difficiency, Boredom, Establishing pecking order, No momma to keep them in line, and just being chickens. I think you are on the right track and I hope you get them lined out.
I like the classical music Idea. I might give that a try sometime. More for me than the chicks. Ha Ha.