Laws of physics.
Short answer: sudden deceleration caused by impact with immovable object at the "wrong" angle.
The laws of physics contraindicate any combination of:
consumption of axcessive alcohol and/ or other drugs;
poor judgement;
low level of coordination;
youth, inexperience and a delusion of driving adequacy;
medical events such as heart attack, stroke or seizures
whilst operating machinery such as motor vehicles.
Eliminate, control or ameliorate the effects of all of the above and you will reduce the road toll by about 90%.