Wild-ass guess: does adding the cold water to the very-hot water cause a rapid convection? You’ve added hot-over-cold, an unstable state. The cold tries to reach the bottom of the cup, and the hot tries to reach the top, very rapidly - and goes a bit too far due to the speed it’s travelling.
super heating occurs when the water doesn’t dissipate the heat put into it, which could happen if it doesn’t boil. having what a person might think would be nucleation sites doesn’t mean that it has to boil, they may not be having that effect at that time for whatever reason.
the temperature has to be measured to know if it has super heated.
But I’ve told you that it was boiling both times. And was not so hot that it kept boiling after the microwave stopped. I would say that it started to boil about 1-3 seconds (three at the most) before the microwave turned off.
But no, I haven’t tried to repeat the experiment; maybe I’ll bring in my meat thermometer and give it a try.
No idea why this is happening. But it this a ceramic or paper cup?
Ceramic. If it matters, the cup itself does not heat up significantly inthe microwave. just to the extent that heat transfers from the water.
This still sounds like superheating. The active boil stops, but the water continues to heat at a slower rate after taking it out of the microwave. Maybe parts of your mug get hotter than you think. I can’t explain it. Somehow, the tea must be getting superheated after the microwave has stopped. Since the active boiling has stopped, areas in the cup remain still and superheat. When you add the water, the superheated regions are agitated and it boils.
That is the best explanation I can come up with.