Bald butts on several chickens...help!

I'm having feather problems with almost half of my girls (3 out of 8); they have suddenly (within about 2 weeks) lost most of the feathers on the back half of their bottoms, and 1 of those ones is almost completely bald on her stomach as well. They are 6 months old, so I don't think it's possible to be molting already. 1 of the 3 looks like her feathers are maybe starting to grow back (can see the shafts through her skin), but the other 2 look as bald and soft skinned as a baby. It also looks like the worst one has a spot on her abdomen that was gashed and bleeding at one time. I'm concerned this could be a serious problem. In case anyone wonders, we give them organic layer feed as well as oyster shell. Can anyone help? I'm attaching photos.