Lack of B-12, protein, not enough space, excess noise or other stress related situations, a sick bird can all cause feather plucking. Find out who the culprit is and separate it from the rest of the flock. Or you can start by adding a good quality vitamin-mineral powder to the water, and keep them on a good 16% or more layer feed. Some folks mix half gamebird pellets with layer pellets to boost protein levels. Anti pick topicals like Hot-pick, or vicks vapor rub put on areas affected will repel the picking. Avoid feeding scratch and extra treats for awhile. People who continually feed vegetables see this problem more often. If they have access to grassy range areas, that is good for them. If they don't have access to grassy range, baby kale is a good treat due it's high mineral and vitamin content.
Also, if you don't have a good ratio of hens to a rooster, feather loss along the back is normal from the rubbing of the rooster's keel bone whenever he mounts his hens. It is always a good idea to have at least 6-8 hens per rooster from my experience over the years.