Research studies on frontal impact car collisions aren't very common. Based on data collected by insurance companies soft tissue injuries are more common with low-speed frontal collisions than we think. In fact up to 70 percent of the people in frontal impacts have neck injuries.
Now a new study from the Biomechanics Research Lab at Yale University has some information to offer. They studied the effect of frontal impacts on soft tissues in the cervical spine. They found even low impact collisions can cause enough strain on a ligament to snap it. This occurs when the force of the impact is greater than the physiologic strength of the ligament.
So it's possible there was nothing wrong with your ligament before the accident. Any past accidents, injuries, or strains could increase the risk of rupture with a low-impact accident.