Final answer:
Cave-ins are the events that cause the most fatalities during excavation work.
Step-by-step explanation:
The event that causes the most fatalities during excavation work is cave-ins. Cave-ins occur when the walls or ceiling of a mine or excavation site collapse onto workers, trapping and potentially crushing them. This is a significant risk in underground mining and can result in serious injury or death. According to the U.S. Mine Safety and Health Administration, cave-ins have been responsible for a significant number of fatalities in the mining industry. Other hazards such as falling equipment, explosions, and gas leaks can also pose risks, but cave-ins are the most common cause of fatalities during excavation work.