In regions with colder winters, it's typically a function of freeze/thaw cycles. Water gets inside cracks, freezes, and the expansion breaks up the pavement. Th

In regions with colder winters, it's typically a function of freeze/thaw cycles. Water gets inside cracks, freezes, and the expansion breaks up the pavement. Then it thaws and softens, and traffic knocks bits loose. Then more water gets in the now bigger cracks, it freezes again and the cycle repeats, but a little worse this time. Then again and again.