Ducts may become blocked when milk flow is interrupted and milk is not adequately removed from the breast, for example if baby misses a feed the breasts may become engorged which can contribute to the problem.
It's also possible that you may experience a clogged duct as a result of a milk blister (also known as a milk bleb), since a small amount of milk will frequently build up behind the nipple pore when a blister forms into a white spot.