In addition, since you do not have any roosters, do you have any hens that are quite dominant and mount your other hens? In the absence of a rooster, sometimes a hen will 'take his place' and will go so far as to mount other hens! I had one that did that in the period of time when I did not have a rooster. She pulled the head feathers the way a rooster would. She went through all the 'motions' except actually touching cloacas.
Otherwise I would also suspect molting (they often do this time of year into the winter) and/or mites as others are suggesting. I hate to use insecticides around my birds but in the case of mites I would also use a product like poultry dust or permethrin, because they are the most effective, and over time mites can cause serious problems in otherwise healthy birds. If you decide to treat for mites, be sure to clean your coop well and work the insecticide into the appropriate areas such as roosting boards, etc. There are a lot of threads on here about this. Use caution about spreading these products outside, especially if you have gardens or apiaries, as they are very toxic to bees and other beneficial pollinators.