I don't think it is molting (we have one hen missing all her vent feathers like this that my kids have taken to calling "nakey bum"). Personally, I am noticing that after several months of thinking she was molting, treating her for lice, watching for a feather picker (which is unlikely since there is never any blood, scabs, scratches, or wounds that might indicate cannibalism), waiting for them to grow back (no signs of new shaft tubules at all for months), the skin becoming very fragile dry flaky, and now finally watching her vigor decline, that it is an infection. My guess is yeast: I am going to try miconazole nitrate cream for a yeast infection. If that doesn't work, I will try an antiseptic cream in case of a bacterial infection.
If it was a molt, it would be over and done with far sooner than this is taking, and with zero sign of feather shafts regrowing that usually show up quite quickly on bald spots during a normal molt.
Now perhaps it started with the lice, for which she was successfully treated, that got things started, but that can cause secondary infections. Will let you know how it goes after medicating her poor naked bald flaky scaly rear end.
I checked during the day and also at night. I do believe they are molting. The feathers are starting to grow back. Didn't notice till I really got them and looked close. A lot of the girls that had naked bottoms have grown feathers back already. I think also sat and watched them for awhile yesterday and they weren't pecking at each other while I was out there. Don't know about while they are on the roost though. 
