Microwaving M&Ms produces sparks
Off the top of my head I would say the sparking and popping is unexpected: usually a significant of water content would need to be present for this kind of behavior, as water conducts electricity and everything else not so much. There is water in peanut butter, but it is around 1-2% and is emulsified with fat, and that should reduce the conductivity substantially. But, the voltages inside a microwave oven are quite high and could force a conductive path through the emulsion to cause sparking. 

My best guess is that whoever makes M&amp;Ms (can't remember now) changed their peanut butter formulation and used slightly more water. It is batch specific?