Hot air expands, and conversely cool air contracts to take up less volume. When you open the door cool air pours out and warmer air rushes in to replace it. When you close the door the warm air in contact with all the cool stuff will cool down, lowering the pressure in the contained space.
This pressure difference means you need to pull against some outside air pressure to open it up. The seal isn’t perfect so air will gradually leak in to balance things out, making it easier to open.
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