The question posed in the title may seem reductive or rhetorical, but it is a serious question. Motor vehicle accidents claim tens of thousands of lives every single year in the United States, but they happen for a wide variety of reasons. Sometimes, a driver is drunk and causes a crash. In other cases, the vehicle breaks down or a medical emergency causes the driver to lose control. In between these two possibilities are millions more. Every car accident case is unique in this regard.
But there are some common threads that we see across all car accidents, and a study produced in 2008 by researchers at the University of Michigan discovered some of them.
One very common reason is following too closely to someone while you are driving. Also known as "tailgating," this annoying behavior (for the trailed driver) leaves the trailing vehicle almost no time to brake or stop should traffic suddenly change. It can lead to a rear-end accident, one that is the fault of the trailing vehicle.
Another common reason is sleepy, fatigued, or otherwise drowsy driving. This accounts for roughly 7 percent of all car accidents, and it is involved in roughly 21 percent of fatal accidents. Please pull over if you are feeling too tired to drive.
Yet another reason, and an avoidable one, is the rolling stop at a red light prior to making a right turn. This insidious behavior is lazy, and it leaves the rolling, turning driver without much time to analyze and see what is going on to his or her right.
Source: Slate, "Anatomy of a Car Crash," Steve Casner, Nov. 21, 2017