You would need to look at the weather patterns. As it turns out there is a reasonably steady movement of atmosphere diagonally up and to the right from the gulf of California into Maine. If you have been watching this year's weather you can see it in action. What is more salient is that moist air coming up from the Gulf of California begins to condense out into rain in the plains. The transport mechanism is fairly well understood and well documented.

That said, the film was susceptible because it was packed with corn husks over an extended period of time. The combination of sensitivity, and extended exposure, led to the film fogging.