Look over the bird that was attacked well, give her a complete health check. Look at droppings, are they normal or not, is she eating and drinking well, look for lice/mites, any injuries, how is her weight, any respiratory symptoms, etc. Often times a bird that is exhibiting weakness or illness will get attacked by the flock. It's survival instinct in them, a weak bird draws predators so they try to drive it out. The symptoms may be very subtle and missed by a human, but the birds know. If the attacks are continuing then I would put her in a wire crate where they can see her and she can see them, but remain safe, until you figure out what is going on.