I recently had a very different experience putting a broody back in with her old flock. For hatching and raising her chicks, she was in a pen that adjoined her old pen. There is just chicken wire separating the two pens. When it was obvious she was done raising her little ones, I let her free-range with her old flock. One of the hens tried to kill her. So I gave it a few more days, then tried again. This time they seemed fine, so I let her go back to her old koop for the night. For a couple of days everything seemed fine, then one morning I opened the egg door and found her all ripped up. Apparently the other hens (there are 3 in that coop) decided they didn't want her in there any more. She was very badly injured and I wasn't sure she was going to make it. Once she was doing well, I let her out to free-range. Unfortunately, one of my daughters opened the other coop not realizing that she was out and the 3 hens tried to kill her again - my daughter caught them red-handed. So, back to the hospital cage she went, and she's doing ok at this point.
I have no idea why these hens would attack one of their own flock, especially since they have been right next to each other during the raising of her chicks.
I haven't come across anyone else with this kind of problem, so hopefully it's just a fluke, but my suggestion would be to watch your girls carefully to make sure they are just re-establishing pecking order and aren't trying to kill your broody.
Lori