Many causes. Inverted nipples maybe natural. Maybe due to previous episodes of infection/mastitis, physiological or secondary to an underlying malignancy. If you have noted inversion of your nipple, you need to have an evaluation by a health care provider experienced in breast care. Breast cancer needs to be ruled out.
It is developmental. This is quite a common occurance but it is of no serious consequence. If you have anty serious concerns about it, you should go see a plastic surgeon and ask if he has any corrective procedure that he/she can offer you.