The stack effect refers to the way air travels through your home. Hot air rises, so as the air warms it travels up and through the house, before eventually leaving via the roof. As the old air leaves, new air rises up to fill the space left behind. Air circulation should be good for the house, as it regulates temperature, and removes impurities.
However, in a house already infected with mold, as the air travels up the mold travels with it. When old air circulates out of the house, it’s replaced with the new, rising air from the basement. This is the air that’s contaminated with mold spores. As it rises, the mold will rise as well.
So, even if you know you never have to open the door to the basement, the air, and with it the mold, escapes anyway.