We just got a little wet here,
Hate to see any rain delays on the last couple races. it might get in the fuel. ;-)
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.