One of them seems to clearly be molting so when I noticed her naked backside it seemed like no biggie. But now 3 out of 4 of my hens have reasonably large naked spots on their butts! One of them molted in the fall...so it would surprise me that she would be doing that again (or maybe that is not unusual?)
It does not seem like anyone is picking/pulling out the feathers, in fact the one without a naked butt is the least likely to be an instigator of a problem.
So after all that. What might I be looking for that would be causing them to loose feathers only on their butts? The spot is below the vent. I don't see anything obvious...obviously..or I wouldn't be asking!