I had just written a reply about how superheating is unlikely because the water would have to be pure (i.e. distilled), but I looked it up and it turns out that's a myth! So exploding superheated water is indeed a potential risk from microwaving

I'm always on the lookout for it (if I see bubbles, I know it's safe), since I regularly boil water in the microwave, and I've even tried to superheat water just to see what it was like, but I've never actually succeeded at it. My understanding is that it's more about whether your container has nucleation sites for bubbles to form than about the purity of the water. My pyrex measuring cup must have plenty of scratches on the inside.

try boiling it twice. once, cool a little, then try it again.

Thanks for the tip, but no luck; it just started boiling again.

but did it start to gradually boil, or boil sort of all-at-once?

Only if you use far too much power x time.

You know, I've boiled water in the microwave and then forgotten it (I want a nag beep)...

So I heated it a second time and it got to boiling point violently all at once.

I think it's because all the air is boiled out of it the first time, which agitates things as it reaches 212f/100c