I have been placing my lager wort in the fridge overnight to get down to my desired pitch temp. This time I opened the fridge to see my airlock bubbling... I understand about possibility of contamination, I always leave my hydrometer out and covered as a type contamination test, and no activity there. My assumption is the cooling wort would create a low pressure atmosphere, so why would it be bubbling?
In sort the gas in the fermenter has higher volume pressure than atmosphere in the fridge. This can only be two things.
The wort or gases were colder than the temperature of the fridge when the airlock was put on, and this is just expansion from temp increase.
There is an organism or other chemical reaction generating gases.
Keep in mind there are conflicting opinions if some sanitizers actually kill yeast like starsan for example. Starsan works by rupturing the cell walls of bacteria, yeast have rather strong cell walls in the microranisim world. It's possible that some yeast from the last fermentation in your fermenter was still there and triggered the prepitch fermentation.