A little while ago, I took a scallion, washed it, cut it up into pieces and put it in a dish in the microwave oven with no additional water and set it for two minutes at the default setting of full power. After a minute or so, I heard a buzzing coming from the microwave and then saw what looked like flame. I stopped the cooking and took a look and saw that some of the thin stems of the scallion had caught fire. I suspect that after the stems got dry enough, there was somehow arcing between the walls of the microwave and the stems. I know enough not to put metal in the microwave and I've always considered it safe to put vegetables in it, even if I don't add water.
Does that kind of arcing really happen? Or should I seek some other kind of explanation, e.g. that scallions, like leeks, sometimes carry within them a little bit of dirt, and that there was arcing between the walls and the dirt?