Single car accidents seem to result in more fatalities than other types of accidents, so generally, we can't ask the driver what happened. We must reconstruct the event by looking at marks on the wrecked car and the road and other pieces of evidence collected from the scene.
Often, single car accidents are the fault of product defects which can cause serious injuries or death. Badly manufactured tires that de-tread, seatbelts that don't work, door latches that fail when they shouldn't, and roofs that collapse and intrude into the occupant space are only a few of the defects that have led to severe damage. 80% of my product liability claims are single-vehicle accidents. The red flag that usually signals an investigation pops up when the occupant receives catastrophic injuries inconsistent with modest car damage.
One case I remember is the death of a woman in a fairly minor wreck. Her door popped open and she was partially ejected, held in place only by her seatbelt. Her head and torso extended far enough outside the car that when it rotated from the impact against a tree, the open door slammed on her. After I filed suit for the door latch failure, the manufacturer conducted a crash test in an attempt to show that its latch would hold the door closed at the speed and angle of the collision.
I inspected its test vehicle on hot asphalt under blazing summer sun outside Phoenix because the manufacturer would not allow me to use its air-conditioned test facility a hundred feet away. Careful examination over some hours revealed that its test was conducted with the door being bolted to the adjacent pillar. The case settled highly in favor of my client, the driver's husband.
So, the next time you hear about a single vehicle accident, don't assume that the driver caused the wreck or the injuries that occurred. Investigate, especially if the victim is someone you care about.