I'm not sure if you've ever experienced a negative pressure case before, or positive/negative pressure differences in a pressurized building (condominium buildings are set up to pump outside air through an air exchanger into the hallways, which then pushes air through an apartment's front door and into the unit), but those two exhaust fans are pulling more air than the front fan can provide.
If you imagine water instead of air, if the front fan is pushing more water in, water will then escape through all the other case openings as the two other fans cannot pull out the same amount. The opposite is, if the two other fans are pulling out more water than the front fan can push in, it will suck water through all the other case openings. As we are dealing with air instead of water, the flow is the same, however when dust comes in with the air it will find places to settle, creating a dustier environment than you would have if air was being pushed out through cracks and crevices. You would have to complement this with dust filters to further decrease dust, however.
Edited by Atokade - 7/11/13 at 10:12am