Water Exploded in Microwave
Joe is essentially right.   Bubbles form in a liquid at what are called nucleation sites -  small irregularies in the container or in the liquid itself.   If you look at the bottom of some beer glasses, there are little nodules (often in the shape of the brewer's logo) that nucleate bubbles of the CO2 that's dissolved in the beer.   Much the same occurs with water - to nucleate bubbles of steam requires nucleation sites and, without them, the water can superheat to beyond it's normal boiling point of 100 C.   When you then disturb the liquid, and so provide nucleation sites, a whole lump of it can turn to steam in an instant.

Permanent damage to the oven is possible but I'd leave it for a few days and then try again.