From the engineering perspective, Air Conditioning is literally that--conditioning the air for use, to ensure freshness, humidity, and temperature. HVAC engineers study this, and apply it.
They provide ductwork for allowing air into the building--continually, not just through doors and windows. They treat it by adjusting the temperature, humidity, and by filtering. Please note--this is a continuous process. if the mechanical system (fans, etc) break down, it doesn't just stop. Warm air rises--and the heat load in any building guarantees that the air will get warmer as it enters a building (Cooling is required even in the coldest weather, thanks to all the people, lights, computers, and other equipment)--and thus it will exit through the upper vents. There is no mechanical blockage, so air will continue entering the building, through the intakes--it just won't get conditioned.
Add an intense heat source, such as a fire, and the resultant broken windows--the air will come in even quicker. Add a great gaping hole where an airplane came through the side of the building, and you have a tremendous addition to the inflow of air, and the intensity of the fire will increase, especially if it is not fought (pumps failed due to lack of electricity/ pipe breakage) and it breaks out more windows. The building acts like a big chimney (try the experiment some time--with a fire in your fireplace, take a page of newspaper, loosely crumple it, light it, and put it as high up the chimney as you can. Listen to the roar, and watch the fire get hotter down below)
the phrase "self eating watermelon" begins to make sense at this point.