Okay time for nerdy chemistry explanation.
The matches when put into the microwave would have lit. Obviously this makes fire. That isn't what's cool. The gas from the Fire gets excited by the microwave radiation. This creates plasma. HIGHLY ENERGETIC PLASMA. <----- DOES THIS NEED MORE EMPHASIS?
Anyway, since he put two fricking boxes of matches. The burning would have caused a minor vacuum depending on how tightly sealed the microwave was. Add High Energetic Plasma and heat in an enclosed space you can get an awesome youtube video.