AKA Boeing did not train, guide or oversee its people well → Workers skipped the process meant to keep track of bolts and hardware → The bolts for the mid-exit door were never put back → At 14K feet, the door blew free.
Physics question: why do I know (maybe wrongly) that in flight the doors are sealed shut by the difference in pressure ? And if so, why did it not work in this case ?
I don't know the answer for this specific case, but the pressure is high on the inside and low on the outside. If you have a hinted door that opens inward, then pressure will keep it shut. If you install the door wrong and forget to attach the hinge properly, it could be blown outward.
This is why the door assemblies are referred to as "plugs" : The door is designed with special hinges & joints, such that opening the door involves first pulling the door inwards, rotating it partially, and then finally swinging it outwards. If the door is not pulled inwards first, the shape of the door (as a "plug") prevents it from swinging out
