Answer: "When you open it, it fills with warm air. Once you close it, that warm air rapidly cools and therefore contracts, causing vacuum pressure inside." @eatredmeat

"You may have noticed it too: re-opening the door of your refrigerator can be really hard immediately after you close it. The door seems to be sucked in. This happens because the warmth that gets inside the refrigerator while the door is open shrinks and creates a vacuum effect."