What is the most likely reason DOT prohibits operation of the vehicle’s engine during loading and unloading of explosives

What is the most likely reason DOT prohibits operation of the vehicle’s engine during loading and unloading of explosives

The DOT regulation at 49 C.F.R. §177.835(a) requires shippers and carriers to load and unload explosives from a motor vehicle only when the engine is not operating. What is the most likely reason DOT prohibits operation of the vehicle’s engine during these processes?