Increased sensitivity would be due to the piercing itself pushing tissue from the nipple out, causing a slight swelling and therefore causing nerves to be stretched slightly more than normal and therefore more likely to trigger a sensation due to 'leaky' cell membranes in the pain and touch receptors (the latter of which are stretch receptors that detect the amount of distortion on the membrane to trigger an impulse).

Decreased sensitivity is probably due to the piercing damaging nerve receptor clusters in the nipple.

Either can happen when a piercing is done and it is almost impossible to predict which or ensure one happens more often than the other as how and where your nerves grow is different for everyone.

This is based on a few minutes thinking about the situation and applying my physiology knowledge...