My DD has been doing this for a few weeks now, although I remember she would do something similar (less scratch-y and more rub-y) when she was a couple months old. I think mostly it's just something she does, although she does have pretty sensitive excema, which appears a lot on the back of her neck (I just realized a few days ago it was from the velcro on her bibs! No more bibs for us!). She has a little bit of dry skin on the back of her head, but since I can't put lotion on it (as it's covered in hair, lol), I think it might be a little itchy. Whenever she does it when I'm wearing her, I go "Do you need mama to scratch your head?" and then I "scratch" the spot (I really just use my fingertips, but I think it helps).