People have been putting things in microwaves since they've been invented, and some of those things, should have never gone in a microwave. But why do some things react so violently when put in microwaves? As far as metal goes, a microwave moves around charges in metal, and can result in a spark, possibly causing other things in the microwave to catch on fire. It's also not a very good idea to microwave an airbag, because they have the potential of going off. There isn't much scientific research inot this one, but if I had to take an educated guess, it probably involves the previous issue of metal in the microwave, possibly causing a fire to start inside the airbag, resulting in a pressure build up, and engage the airbag.