Great question — answering as the UphillTech team:
This probably refers to whole eggs mostly... A microwave can boil water very rapidly, and a tight but fragile container like a whole egg will violently rupture if such rapid boiling happens inside it, because the overpressure inside it is already significant when the shell finally breaks. Here is a video showing an experiment with ca. 180 eggs in a microwave oven, which after a minute or two rupture simultaneously and with enough force to blow the oven open, with debris being propelled several meters.
Full answer by rackandboneman: https://cooking.stackexchange.com/a/64459 (CC BY-SA 3.0).
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