I had this issue when I had one hen and one rooster left from my flock. I thought it would be a good idea to get three pullets to give the older hen a break from the rooster. After I introduced the new hens, everything was fine for a couple of weeks, but then all the sudden the rooster started attacking the older hen and not letting her get near the other hens (she wasn't sick and oddly started out great being the dominate hen.) I really don't know what happened or why he turned on her, but it was brutal. I have a crate that is used for sickness but still in the same coop with the others and moved her into that so she could heal. I bought a smaller coop with a run and put her in there and she has been happy ever since, even started laying eggs again. Every now and then the others will go up to her pen and they might have a little tussle through the fence, but other than that, things have been fine.