My suspicion is that he filled the cans without removing them from the vehicle and placing them on the ground. Gas flowing through plastic can generate a significant static charge. When it dis-charges the spark will ignite the fumes. You should always place plastic containers on the ground when filling them.
My thought too. Place them on the ground and touch the nozzle to the container before pulling the trigger.
I once watched an airplane burn to the ground because of static. I grd everything.