Usually from massive internal organ damage. crushed spleen, kidneys, liver, and so forth. Internal bleeding, leading to hypotension and eventual loss of blood pressure causing the person to crash and subsequently expire. Also, things like a pneumothorax, or a broken rib piercing a lung and the person suffocating, or a blood clot going to the heart or lung. Many things that can kill a person if, say, hit by a car, that doesn't cause a broken neck or head trauma to the extent that would cause a fatality.