If you regularly carelessly nuke stuff, you'll know that in some cases you'll come back to warm food, in some cases you'll come back to shriveled, dried food, and sometimes you'll come back to a fire in your broken microwave. What difference caused these events?
Microwaves, despite being around for a half a century, haven't lost their sense of mystique. Sometimes you come back to them and your food is hot, sometimes it's warm, and sometimes, when the tray gets stuck, it's a patchwork of hot and cold. Then again, sometimes, it has spontaneously burst into flames. This happens especially often when people microwave damp cloth or damp paper in order to get it dry.
But wait, we know how a microwave works. The electromagnetic waves that are sent back and forth across the little illuminated cavern inside the microwave are meant to excite water molecules. And as much as you excite water molecules, they don't catch on fire. What should happen is the water getting excited . . . until it just plain boils off the food. At that point, there is no longer any more water inside the food and the waves don't do anything. How could anything scorch?
For one thing, plain water isn't too much of a problem. It's when electrolytes, like salt, are in the water that things begin to heat up. Salt lets water carry an electric current. This isn't a problem for plain water or really wet food. No fires are started when anyone nukes salt water. The problem is when two areas have a lot of salty moisture in them and there's a dry spot between them. Electrons from one area could be yearning to go to the other, but the space between them won't let it happen. Until the electrical potential between the two areas gets too great. Then - crack! - a spark arcs between them, the same way lightning arcs through the non-conductive air between the sky and Earth. (And the same way metal will spark in the microwave.) And just like lightning, this spark creates a lot of heat. That heat, which is being created in a dry area in the food, or cloth, or paper, starts burning things. So when you come back to the microwave to find your food scorched, what often happened was a 'lightning bolt' between two wet areas in the food.
Image: Heb on Wiki Commons