It's just what they do to the hen at the bottom of the order, and there isn't much we can do beyond making sure they have adequate space, and enough food and water. If it is really bad, and/or if there is one bully hen in particular, you can isolate the bully to take her out of the equation and see how things settle. When you reintroduce her, another hen might have taken over the top spot and might not be as ruthless. More than likely, she'll just bully her way back to the top again, tho. And unfortunately, taking the injured one out (which I understand you had to do) only reaffirms her place at the bottom.
Having a rooster can also help settle the hens, as they generally defer to the rooster as the top of the order and the roo can help settle disputes.
Injuries notwithstanding, I've had to remind myself that this is normal chicken behavior and that I shouldn't personalize it. It's hard, though.