We were parked behind this tanker when I noticed a coil of green and yellow wire on the back. That's the Earth wire. The process of driving on insulating tyres causes charge to build up on a vehicle, giving it a high potential. Earth has a potential of 0V. This can cause sparks as the potential difference causes electric charge to flow, with maybe explosive consequences if the tanker contains powder or flammable liquid. The Earth wire must be connected to the ground before the vehicle is emptied.
