Low fluid level could be one cause, more likely condensate from extreme temperature conditions or a missing dipstick. AUTOMATIC TRANSMISSION FLUID WILL LOOK FOAMY IF CHECKED WHEN WARM. THIS IS NORMAL. YOU SHOULD HAVE A QUALIFIED TECH INSP/CHANGE THE FLUID IF IT HAS NOT BEEN DONE IN THE LAST 30K MILES ANSWER Some oil additives have been known to 'foam'. So if you added an additive to your tranny that could also be a cause. ANSWER Sure there is not some water into the trensmission?. Get qualified help to inspect.