New researches show that clogged duct actually starts outside the duct - the breast tissue swells, leading to narrowing of the duct, and thus slow movement of breastmilk during pumping and latching.
You may ask what is the cause of breast tissue swelling? Oversupply was pointed out as the biggest risk factor. Think of the breast tissue as a muscle. When a muscle is overworked, it gets swollen, inflamed, and painful. When breast tissue is overworked from producing high amount of milk, it may also get swollen, inflamed, and painful.