Do you think that maybe they are trying to avoid the drake and can't escape? Do they have ample space to escape or are they trapped in a pen with no way to get away from him? I have seen a lot of rough behavior, but never feather pulling like that. I have one female that has stayed with my drake that currently is laying eggs, but the other females escape from him. It could be that he is so forceful because they reject him, so for that reason you could justify getting a new drake if the females reject him.
For example, my drake mates underwater and drowns the female, dunking her head underwater for up to 10 seconds, then pulls her head up. But they are a pair, she stays with him. The other females arent laying eggs yet, or they are scared of him because my ducks split up most of the time.