If I remember my physics for philosophy major course, certain plastics absorbs microwave energy at differing levels, or rather, they are affected by microwaves depending on chemical composition and whatnot. Microwave-safe plastics are a-ok to use in microwaves, but the cheap stuff from the dollar store "flame-up" spectacularly (screaming fiance and smoke detector usually ensues soon afterward). I was actually surprised that the plastic on the 360 didn't catch on fire, but was somewhat amused they set the controller on fire, but I wonder what happened on the inside....
Oh, and they should consider checking themselves for lung cancer in a couple of years down the road.