The best advice I can give is, first, to really look them over to make sure they don't have lice or mites. If you don't find anything, then just sit and watch them for about an hour. Then, you'll find out if it's a hen or the rooster that is pulling feathers.
It very well could be from the rooster....Sometimes they do grab hold of the hen's feathers on the crest to hold them still while mating. They usually don't grab onto the tail feathers though...Sometimes the backs of the hens will look worn or scraggly from the rooster mounting them.
This make sme thing a hen could be doing the feather pulling. A clue for you....Is there a hen with all her feathers? She may very well be the one pulling. I had a similar problems like that to discover a NHR hen was pulling the feathers out of each of my chickens, just above the tail. Even the rooster was getting a bald spot. I got rid of that hen. I like my chickens to look pretty. All the feathers eventually grew back.
Good luck and I hope you figure things out,
Sharon
