Phrakhanong police in Bangkok went to the scene of an accident in Sri Nakharin Soi 40 opposite Seacon Square where a motorcycle taxi rider died.
Witnesses said that as the victim and a tour bus were passing each other a door to the luggage compartment opened all by itself.
Sawat, aged about 60, died at the scene after suffering a severe neck injury and subsequent blood loss.
The bus is privately owned and police are having the door mechanism examined and investigating possible negligence.
There were no passengers on the bus at the time.