Poor Derp. I have noticed that when a hen emerges from her broody obsession, she is often bullied for a while, and little Derp's squatting seems to be an invitation for the other hens to gang up on her. I wonder if dividing the run with some chicken wire to give her a section with the pullets would help. (How old are they now?) The other hens would be able to see her, but not hurt her. And Derp would gain some confidence around the chicken run.