Most often the stack effect (warmth in the house causes air to rise & leak out the attic which sucks in replacement air down low- like a chimney), but also potentially overpowered range hoods, bath fans, clothes dryers, open flue gas appliances, wood fireplaces, etc.

Incidentally this is why there are limits on range hood CFM without makeup air (and there are broader recommendations on overall house design from groups like ASHRAE for air quality management). There are also whole-house leak tests in some regulations, e.g. Title 24 in California.

Typically it's around 300CFM before you should consider a mechanical make-up air device (gravity activated or a makeup air fan tied to the hood activating).