Ok just wanted to make sure it wasn't a coming into sexual maturity thing.
I'd advise taking the female in for a fecal exams. When a cage mate starts going after another's tail or butt area it can be an indication of parasites/worms or some other health/medical issue going on. You may need to do 2 rounds of fecal testing 30 days apart to rule out parasites or worms, they don't always show up in the first test.
I asked about diet because if you feed a diet that includes bee pollen the vet needs to be informed of that and that they can cause a false read on the fecal tests.