Poor guy learned the hard way.
Microwaves are not to be fucked with. Microwaves are produced inside the oven by an electron tube called a magnetron. The microwaves are then reflected within the metal interior of the oven where they are absorbed by food.
When a magnetron bombards another magnetron with microwaves it produces a feedback loop that can result in an explosion. This phenomenon was first recorded in 1998, the same year that The Undertaker threw Mankind off Hell In A Cell, and plummeted 16 ft through an announcer's table.