How old are they? As pervious posts say hormones could be kicking in. We have 3 pullets: 8 month Rhode Island, 6 month Barred Rock and a 5 month Australorp. My BR has been laying for 2 months now and as she grew and her comb and waddels came in (right about the 6 month mark) she woke up one day grump and pulled my RI (oldest) from her nest box. Causing a minor cut on the RI comb. They scuffled the next few days and I let it go until I felt it was to much. Then the BR was placed in a "travel crate" inside the coup while we arent home . I let them in the yard all day and as evening approached I took the aggressive bird and placed her in the "travel crate" until the others were on their roost for the night, then I placed the aggressor with the others on the roost. Chickens have poor night sight and get a little loopy the darker it gets. This usually means they sleep together just fine. They woke up the next morning and were fine. Have been since. Try "chicken jail" for the aggressors. This advise was given me and it worked.