When heated, air expands. Now when you take the tupperware out of the microwave and it cools down, the air inside cools down again and contracts, and 'sucks' the lid inwards.
Actually, that's not quite right - it's more precise to say the air outside presses the lid shut. The bodies of air outside and inside seek to equalize their pressure, and normally the tupperware would just shrink / contract until it's inside pressure is equal to the room pressure. But since the container is somewhat rigid, it can't shrink enough and there remains a pressure difference. This causes a force acting on the lid, keeping it shut.
cooling down => volume shrinks
cooling down, but forcing the volume to be constant => pressure goes down
inequal pressure => force towards the lower pressure