Why did my banana catch fire in the microwave?
It's caused by the high amount of potassium in the banana. Microwaves react with metals, bouncing off and cause arcing. You can even create a cool light show by putting a raw peeled banana in the microwave. Don't worry, it won't explode, but it will make a mess, it's also harmless.

This can also happen in some frozen vegetables depending on the soil conditions they grew in. As noted in the linked article, other high concentrations of metals such as magnesium, iron, and zinc can be the responsible mineral.

Momentary brief sparking is harmless and won't harm your microwave, but the USDA recommends turning off your microwave if you see sparks nonetheless. If it does start a fire, unless it's a very small fire, do not open your microwave door. The safest thing is to turn off the microwave and let the fire burn itself out by consuming all the oxygen. Opening the door could create a dangerous backdraft induced fireball. 

See Also:


http://www.scribd.com/doc/1697458/USDA-Microwave-Ovens-and-Food-Safety
http://www.inspection.gc.ca/english/fssa/concen/specif/veglegmicroe.shtml