|My wife makes blueberry pancakes for my daughter daily. Twice recently she noticed and brought to my attention a curious event in the Microwave oven. Frozen Blueberries placed inside a microwave oven to thaw, caused a popping sound and a small flame to appear amidst the blueberries. The flame self extinguishes. There is no apparent damage to the blueberries or the bowl they were contained in. — HA, New Jersey
I think that you've rediscovered an experiment in which people cut a grape almost in half, open the two halves like a book and lay it flat on a plate. In the microwave, the thin bridge between the halves carbonizes and than emits flames. Basically, the fruit pieces or berries are acting as antennas for the microwaves, which drive electric currents through the narrow bridges between parts. The berries aren't great conductors, but they're not true insulators either. Those bridges overheat (like an overloaded extension cord) and burn up. The flames come from the burning bridges.
If you let the flames go on long enough and enough carbon develops, you'll probably start getting plasma balls in the oven (lots of fun, but not great for the oven... you can scorch its top surface because those plasma balls rise and skittle around the ceiling of the oven). Anyway, you can probably find the carbon areas if you look closely enough, but they're no worse than a little burnt toast.