Potholes are for the most part a function of water infiltrating the subgrade. When water that has managed to infiltrate the paved surface freezes it expands as it freezes and forms ice. The pressure exerted from this process starts to break up the asphalt in the surrounding area, allowing for even more water to enter once the ice melts.
After a series of freezing and thawing cycles, the surface will break apart and a pothole will develop. During the winter months the hot mix asphalt plants are shut down due to the cooler temperatures making the use of a cold weather asphalt mix necessary. The patches done with this material sometimes fail. They are then replaced later in the year as hot mix is made available by the asphalt plants.