Q:

How could a microwave launch a cup?

This is something that's puzzled me for years and I'm hoping someone here can offer a theoretical explanation and put my curiosity about it to rest.  About 15 years ago in my bachelor days I wasn't so good about putting my dishes away and left a clean, empty plastic drinking glass on top of my microwave (which sat on top of my counter) undisturbed for weeks. 
The glass was made of thick heavy-duty translucent plastic and gave the cup some heft.  One day when I was cooking something in the microwave I heard a distinct pop and saw the cup fly off the microwave--it fly up a few inches to maybe a foot and landed on the floor.  
What physical phenomena could have caused this?  Static buildup and sudden discharge?  Spring loading of the metal body of the microwave and sudden release?  Heating of air under the cup (it had a small void underneath with a uniform lip that would have trapped some air)? 
Something else?  I no longer own the microwave or the cup so I can't say for sure the microwave wasn't defective in some way, nor can I conduct experiments.  

A:

If was a heavy cup, I would be inclined to rule out trapped air. The larger the volume of air available to act as "thrust", implies the cup was bigger, and so heavier, so I guess that's not it.
A faulty microwave might shake, or if the rotating plate "fell" into it's turntable slot position, as the rotation continued, it might have shaken the machine.
The cup could have been located over the air vent, (which manufacturers  do locate on the top) and gathered enough hot air to lift off.
But the most likely  option imo,  is the sudden flexing of the machine roof, which might have heated up enough.  I think microwaves ovens get hot enough for this and it would be my best guess. They are tightly fitting casings, so it may act as a mini drumhead, that is fairly tense in the first place, then heated air gets trapped below it, and  the top of the oven springs upwards to release the pressure. 

