You exceeded the liquid’s boiling point, and it exploded in the microwave oven, that’s what happened.
Years ago, school teachers used to teach that water (unless pressurized) could never get hotter than its boiling point. That seemed true then, you put a pot on the stove, and it would bubble and boil furiously, allowing steam to escape fast enough to keep the water from getting much hotter than 100 °C or 212 °F.
Microwave ovens, however, changed the rules.
In order to start boiling, water needs hot spots, impurities in the pot surface, or small pockets of trapped air to act as “seed bubbles.” All of these are plentiful in metal boiling pots, but not when microwaving in glass or ceramic containers.
So, while liquids will often boil in the microwave when heated, there are times when they won’t, no matter how hot they get. This is especially true with soups, because often a thin layer of fat floats on top, hindering evaporation. If you’re lucky, this superheated liquid may spontaneously explode in the oven.
If you’re not, it may not explode until you induce an instantaneous boiling explosion by moving it or putting sugar, salt, or a spoon into it. People have been severely burned on their arms, upper body, and face.
So how do you prevent this? Well, the best idea is to not overheat liquids in the first place for example, use the timer instead of the “zap it till it’s bubbling” method. Waiting a minute before removing hot liquids from the microwave isn’t a bad idea either, keeping the cup far from your face in case it blows.
Since kids generally have shorter arms and less real-life experience, they are more likely to be injured; teaching them microwave safety is an excellent way to prevent accidents.