My guess: Underground piping from the tank to the pump has a leak as well as a water line in close proximity. Both under the pavement allowing the pressure to build and water to seep in to the fuel line, which is not pressurized. Only thing left since the tanks tested OK and I really doubt if Sun Oil would deliver bad fuel to a Nascar race where they are sponsors. This would yield the first couple of tanks to be severely contaminated and those there after, not so much.
"Darby dismissed sabotage as a possible explanation, saying too many teams were affected for anyone to have been singled out."
SOooooooo they do think of things like that!