Ever been inside a empty storage tank on a warm day?
85F outside can get over 110F inside after a few hours of direct sunlight.
I would reckon it was a static electric spark, possibly from prior heavy rain that compressed the roof, and/or the tractor trailer tanker was ungrounded when the transfer connector was inserted.
Irrelevant. There should be a grounding wire connecting the source and sink containers Connecting that wire first brings the entire system to the same potential. Since the entire syatem is an excellent conductor, the charge generated by the viscous flow dissipates and appears on the outside surfaces of the system which contains the Earth's surface. Since the system is a good conductor, the charge will quickly recombine though currents in the conductive containers/piping.
Also, sparks do not occur inside of a tank from one area of a wall to another, or from one area of a fluid to a wall. Any field resulting from the charge on the tank appears on the outside of the tank, not on the inside. The field inside is zero, which mans no sparks are possible.
All that can be lit up is the fumes being initially displaced from the tank. ...if they are at a temp above the flash point for a high enough concentration AND there is a source of ignition provided. Since the driver attached the ground wire, there is no source of ignition, except those provided deliberately.