I think that it is the fumes which "siphon" out of the tank when the tank interior becomes hotter than the outside air. After the fumes make it outside to where the air is cooler than what the interior of the tank is, the fumes then roll down the filler neck and onto cab.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission