Are you thinking of the superheating effect? Reasonably pure water in a smooth cup doesn't boil in a microwave. It becomes superheated, and then when moved nucleation sites form causing some of it to flash evaporate into steam and blow up out of the cup, taking lots of very hot water with it. This burns some people making tea or instant coffee.
This almost certainly would never happen in this case as the rice would provide ample nucleation sites.
No, I'm not talking about superheating -- I'm talking about boiling in a sealed container. People do this deliberately with pressure cookers, where the pressure typically reaches double atmospheric pressure; but it can happen to a lower yet still dangerous pressure by accident.
This almost certainly would never happen in this case as the rice would provide ample nucleation sites.