Motor vehicle accidents usually result from a few mistakes, not just a single error. They're complex events. For example, a semitruck might swerve to avoid an animal entering into the road. That might make the vehicle next to them rush into another lane, where their vehicle is hit in the rear. The semi may not be able to regain control because of wind conditions, which then leads to a crash with multiple vehicles.
Weather conditions, drivers' reactions and other external causes can all lead to crashes. Internal problems can cause collisions, too, like being overworked or too tired to drive safely.