The two biggest factors that cause a small crack or ding to spread would be the weather and vibrations. The extreme temperature changes that we experience here in South Carolina can cause the crack to rapidly grow. For example, if your car has been sitting in the scorching heat for hours and then suddenly the A/C starts blasting, that is very taxing on the windshield. The same concept goes for cracks in the winter time. The second factor that causes a small crack or ding to expand would be any vibrations to the car, whether that be driving down the road or hitting a pothole.