what does a microwave do?
it has an emitter that bombards the chamber with microwaves.
water molecules in whatever you put in there absorbs the microwaves, and which is then converted into heat. at a certain point, the water will be vaporized (not evaporated, there is a technical difference in meaning) due to the temperature increase resulting in the expansion of chambers within the dough (of air, and formed steam). and the rapid expansion coupled with the nature of dough in general causes popped bubbles of steam. thus you have "explosions"
sparks should not really fly though.
same deal with margarine.
--
i don't know how "oil" reacts to being bombarded with microwaves.