Backside pecking usually means they are pecking at something - please check your chickens carefully for lice, mites, worms, etc. Do your chickens have access to a dusting bath? If so, is there DE in it?
You can spray the injuries with Blu-Kote from the feed store. Separate bleeding chickens as they will continue to peck blood.
For treatment of the problem:
1. You can try rooster booster pick no more lotion or black salve from the feed store as it tastes bad on the areas after healed.
2. You can get rid of the mean ones or make more pens for the mean ones (even separating for 2 weeks or so will lower them in the pecking order but prob. temporarily).
3. You can try pinless peepers as a last resort (blinders for the perpetrators).
4. Feather picking is a habit. Sometimes it just happens and if overcrowded it is more likely to happen. If you are overcrowded you might try to alleviate that. If not overcrowded and you can't solve it, you might have to just get rid of the mean ones.
In other words, there are usually one or two troublemakers I have found from experience. When they are removed from the flock the pecking/picking stops. I have gotten rid of hens for aggression. I have zero tolerance for aggressive hens (but that has nothing to do with the fact that I'm currently on a chicken break, lol).
You might be able to nip this in the bud with "painting" the affected areas, or not.
Also check to see that your chicken diet has adequate protein.