Pecking between chickens can be caused by a number of things: pecking order, boredom, lack of space and aggressive behaviour. Any one of these is enough to set it off, and once it has started it is difficult to stop. Feathers are very high in protein. Chickens love this taste and come back for more.
Also blood is drawn when a feather is pecked which chickens love. So get it under control as soon as possible. Be sure to check for vent pecking as it is very serious and can go unrecognized.