From Net Doctor
Over-manicuring is a common cause, cleaning underneath the nails with a file can damage the nail bed and cause the nail to grow in the way you describe.
Poor circulation in the fingers.
Overactive thyroid gland
Psoriasis
Fungal nail infections can also lead to this condition but is usually accompanied by areas of thickening and distortion of the nail.
An allergy to something that your fingers are in contact with, such as glue used to stick on artificial nails.