Static is a primary driver. Put excessively simply, if your return ductwork requires an 1" (250 Pa) of pressure, your space will be pressurized by that amount, unless you provide a return/relief fan.
When pressure relationships are important have a supply and return fan with some sort of tracking. In an office or home application with a packaged unit (Carrier, Trane, York, etc.), a relief or exhaust fan can come on during economizer operation to prevent over-pressurization.
