You have seen a match make plasma in the microwave. You have seen a compact disc light up. Now you are going to see why it is dangerous to put grapes in the microwave.
What's going on? Well, I only have an idea for some parts. The first part is speculation -- the sparks are resulting from an electrical discharge between the sides of the grape, as is evident by watching and as supported by the 120Hz buzz that coincides with the grape flaming. It could be that the particular size of the grape relative to the wavelengths of the microwaves in the oven causes the discharge, but I don't know if I believe this yet. What is the mechanism for charge separation?
The discharges vaporize the sugars in the grape and then cause them to combust. The combustion products, if they get hot enough due to the continued electrical arcing, form a plasma (gas where the nuclei and electrons have been ripped apart from one another). This plasma is electricially conductive, and so absorbs microwaves keeping itself warm, and causing the cloud to glow. Since the cloud is hot, it rises to the top of the oven. However, the cloud seems to stay remarkably coherent. Why doesn't the could dissipate over timescales of several seconds?