It could be a rough play/mating thing but it may also be patchy skin/fur from too much protien. I'm more into rats but I know I've had a breeder male that grabbed the females so hard behind the head/shoulder area when mating that he slashed them rather badly (he was soon out of a job let me tell ya!) Keep an eye on them, perhaps take a look at the protien percentage they are eating and see how it goes.