"Birds will show cannibalistic behavior when they are crowded or feed-restricted. Flock behavior and inter-bird dynamics often include aggression of birds toward each other, which can ultimately result in injury. Cannibalism is a separate problem that may occur long after dominance relationships have been established. Cannibalism differs from dominance as it actually causes physical harm. Cannibalism may begin with feather pecking and is usually directed toward the body, toes, tail and the vent area. Prevention of cannibalism is much easier than treatment."
Overcrowding- You need to increase amount of space for each chicken, so they are able to escape being pecked
Not enough water/feed- Be sure all chickens have access at all times, they drink a lot of water so provide extra water bowls with fresh water daily
Bright lights- If you have bright lights in the coop or run, keep the light intensity low to decrease extra activity/cannibalism
Too hot- Keep the coop/run ventilated and cool, hot temperatures aggrivate them
Nutrition deficiency- Deficiencies of nutrients like methionine, protein, and salt will increase a craving for feathers and blood
External parasites- Regularly treat your chickens if they have lice/mites/etc. The parasites will encourage them to peck/injure skin
Sick/small/weak chickens- Remove all sick, weak or small chickens, the dominant chickens will attack them
Hard ground- Your chickens need to scratch, dig, take dust bathes, etc. A hard surface will prevent them from doing this and keep them less occupied. I put mulch, mowed grass (fertilizer free), leaves, and loose dirt. I also spread seeds around for them to find. Do not use gravel or hay (hay will attract mites/lice)
I have not personally experienced cannibalism with my chickens, but I know a few others who have chickens that have hen pecked each other (to the death) because of too small of a space. Chickens are made to roam and forage in order to find food. Even if they are getting enough food for the day, they will still want to forage. If they can't they will get bored and beat up on each other (which is the same with all animals). The main thing to remember is to keep them in as natural a habitat as possible, which will give them space to roam, escape pecking, and be occupied.
This link shows pecked chickens, and a way to treat it:
This link shows how to make chicken "capes" to cover their injury: