The Stack Effect Phenomena:

Hot air rises, right? Well as warm air rises through your home, it will eventually leak out the roof. New air must enter to replace the air that escaped, so the home will pull in air from your basement/crawlspace. Imagine a pulling offensive lineman on a power run play coming from the deep depths of your basement to come knock you down. Scary isn't it? That is how strong and fast the air from your basement can reach your living spaces. In some older homes, air can move from the outside, to the basement, to the roof, and back outside again in less than an hour. These homes are wasting ridiculous amounts of energy on heating bills just to have their "controlled air" leave the home within less than an hour. The stack effect also creates problems because around 50% of the air you breathe on the first floor comes from the basement/crawlspace. If your basement/crawlspace is dirty and contains harmful elements, you will end up breathing it in.

With low air pressure on the lower levels, your house sucks on the ground, then blows air out of the upper levels due to a high air pressure.