When you open the door of a fridge, two main factors come into play that cause things to move and shake inside:
Air Circulation: Fridges are designed to maintain a cool and consistent temperature inside. When the door is closed, the cool air remains relatively stable, and the items inside the fridge settle into their positions. However, when you open the door, warm air from the room enters the fridge, and cold air from inside starts to flow out. This creates air circulation within the fridge, causing items to move or shake due to the movement of air. The movement of air can also create tiny pressure differences, leading to doors or lids of containers popping open.
Disturbance: Opening the fridge door involves physical movement and can result in vibrations being transmitted to the contents inside. When you pull or push the door, it might cause some items to shake, especially if they are not tightly secured or balanced.
Once you close the fridge door, the circulation of air inside gradually reduces, and the temperature stabilizes again. As a result, the contents inside the fridge settle down, and the movement and shaking come to a stop. Any remaining vibrations caused by closing the door would dampen out quickly, leading to a still and quiet environment within the fridge.
It's important to note that some fridges come equipped with special mechanisms, such as dampers or air flow regulators, to minimize the effects of air circulation when the door is opened, which can help reduce the movement of items inside.