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.