that plastic or mica insulator is a cover for the microwave waveguide. at the top of the machine is a hollow rectangular tube that goes to the "cavity" on the magnetron tube. the magnetron is what produces the microwave enegry to heat your food. what happened in your case, a hot spot showed up on that insulator and when the magnetron fired, it concentrated the micro waves on that one spot and ignited it. you can remove the piece and replace it with a thin piece of poly carbonate.