The explosion destroyed both microwaves and created a portal to the astral plane.

Joke aside, the magtrometer can easily explode.

Jokes aside, the magnetron is just a vacuum tube surrounded by a big heatsink. It could implode but the vacuum volume is tiny and the metal inside and outside will prevent glass from flying everywhere, unlike a B/W CRT. There is also a transformer, HV capacitor, fan, a tray motor, a mechanical or electronic timer, lightbulb, high voltage diode and fuse, bimetallic contactors (thermal fuses), safety switches, interference filter and smooth start circuitry. None of this can really explode but will get hot in presence of microwaves, burning plastic and other materials, producing caustic smoke.