Engine Failure,
It's not against the rules, although each company will have its laid down procedures for carrying out refuelling with pax on board.
Near Enuff,
Refuelling whilst disembarking is not a 'no-no'. Certainly in BA it is permitted and both flight crew and cabin crew are, according to Flying Crew Orders, to be aware that it may be happening even without having first spoken to the refuellers ie they might be putting on an 'initial' so many tonnes before getting the final figures.
According to the Company, the ignition source is not known but it seems the fuel hose disengaged from the aircraft, spilt a load of fuel on the ground and it is this which caught fire and scorched the aircraft.
Pontius