All these analogies to potato guns, bicycle pumps, etc. are evocative, but they just don't match reality in this case. First of all, you have huge quantities of material being ejected out of the building on all four sides at the "collapse" front. Whatever "piston" you imagine pressing down on the air must be sealed off from this, otherwise the air would just go where all that other air is going.
This means that whole floors have to fall at once, below the visible "collapse" front, because if only part of a floor fell, the compressed air below the falling floor would rush up and around to fill in the low pressure above the falling floor. Remember, we're only dealing with one atmosphere of pressure here (absent explosives that is), thus any local overpressure should be matched by a corresponding underpressure somewhere else.
So you imagine whole floors dropping ahead of the collapse front, and compressing the air below it, forcing it into elevator shafts and air conditioning vents. This increases pressure in the elevator shafts and air conditioner vents, which distributes that air throughout the system.
Are you guys suggesting that the entire tower below the collapse front had its air pressure raised, that the whole thing was inflated like a tire, and that those two windows on the whatever floor popped because they were the weakest point in an airtight vessel? Is this what you guys are trying to get me to believe?
Just out of curiousity, what is the velocity of an RDX shaped charge at 100 feet?