The packet says remove or pierce film before cooking.
The microwave heats the water, which turns to steam and expands 5,000% IIRC. This causes the film to be unable to contain the pressure difference, unless pierced as per instructions.
On a similar basis - My dear old dad found a lump of lead in the garden. He explained how when he was a boy you didnt buy toy soldiers but the mould and made copies from molten lead. He found an old saucepan and melted the lead and an old London brick imprinted with LBC etc and washed it to make a mould, excited as a child. Then he put the molten lead into the brick, which demonstrated some basic physics. The traces of water in the "LBC whatever" inscription became steam and 5,000% larger. This in turn caused a violent bubble of superheated steam which flicked liquid lead over a surprising area, most significantly including my best friends face. Fatherless himself he had failed to be somewhat reticent about an over-enthusiastic father playing with molten metal and had been leaning in for a closer look. I was leaning well back and only had a shoe and a trouser leg flecked in liquid lead.
On the up side, five or ten years later you couldnt tell, and these days he's a pretty boy with a pretty lady.
