Sudden onset hen-on-hen pecking?

I have 2 Barred Rocks and 2 RIR. The Rocks hatched 5/11. The Reds hatched 3/13. The Reds have always been on the lower end of the pecking order since integration in May. Between the 2 Rocks, hierarchy is well-defined as well, but there's no aggressive/physical behavior between them. There are no roosters.

This morning when I let them out of the coop, I noticed the subordinate Rock looked "fluffier" than a chicken should. Sure enough, all those feathers on the coop floor belonged to her. There's a small patch of bare skin on her back about 2-3" above the base of the tail. She can lay her remaining feathers over it so that you can't see all of the bare patch. The skin is not broken. There's also a fluffy spot on one side, but it doesn't look to be as bare. It looks like someone grabbed a handful of feathers and yanked. I would think that a peck job that de-feathered her would have resulted in some broken skin. I can't imagine after all the time these birds have been together (6 months now) something would changed so drastically that this would happen. Yesterday I noticed that the Rocks (both of them) were huddled in the corner of the coop, which is out of character for them. I passed it off as trying to stay warm because yesterday was our first real cold day, and it was a large temperature difference from the day before. Now, I'm thinking it might be due to whatever dynamics are playing out between the hens. There are also lots of feathers in the coop itself (noticed this this afternoon collecting eggs).

Today all four birds are behaving normally. And pecking order seems to be what it's always been.

I am sure this is not a molt. As no other animals are responsible for this, I'm not sure what to do. I don't even know who's to blame.

My concern is two-fold. I need to figure out who and why if I can. And I need to know if I need to do anything for the de-feathered hen beyond keeping an eye on the bare patch and making sure it doesn't get abraded (and treating it if it does). Is it likely that the Reds are working toward re-establishing pecking order? Wouldn't the dominant Rock have had something to say about it? Advice or insight would be greatly appreciated.