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Some of us are little slower than others. I have been here since the beginning. My member number is below 200 and I have about 13K points. Obviously, it's not a very high priority for me at this point in time.
I first experienced this years ago - without even a microwave - but a brand new tea kettle on a gas stove.
You can look up all the grisly details you'd find interesting, but the basic requirements for this are heating in a brand new container without any internal flaws (micro-scratches from normal use). With no irregularities, the water reaches it boiling point (not enough reason to boil, by the way) and then, added energy would cause the conversion from liquid-to-gas.
But where ? ? ? ? The microscopic scratches are places that lower the energy required to cross that gas/liquid boundary. Suppose there aren't any?
If the container's "flawless" from the point of view of the heating content, then it will just keep getting hotter, even exceeding it's boiling point at the ambient pressure. Then - you move it or the energy content gets high enough it can jump that barrier.
Once it starts - chain reaction - watch it!
This is not related to microwave heating, in particular, but the way and what we heat things in a microwave oven lends itself to this phenomenon.