The reasons are entirely logical. Increased drunk driving legal enforcement, more wide-spread use of seat and safety belts, better roads and of course the modernization of America's car and truck fleet which includes modern safety features such as anti-lock brakes, airbags, and stability control.
Even motorcycle crash deaths went down for the first time in more than a decade. http://www.maryland-law.com/library/motorcyclist-fatalities-on-the-rise.cfm
Nationally nine states saw increases in car, truck and motorcycle crash deaths with those states being rural or smaller ones with a prevalence of weaker seat belt use laws.