content: There's a well-known phenomenon referred to as "bumping" (more scientifically called superheating), which can happen when you heat a liquid in a vessel that is smooth and free enough of defects that it becomes difficult for the fluid to form gas bubbles and evaporate normally. When in such a situation, the fluid superheats and any disturbance of the fluid bulk (or, "bumps", hence the name) will cause the liquid to spontaneously evaporate, expanding violently and often bubbling out of its vessel. I imagine, in this case, that something along those lines happened. When he opened the microwave door, he disturbed the vessel, causing the water to vaporize and rupture the vessel.