I have observed this phenomenon for decades, and always concluded it must be due to a fan in the freezer evacuating the warm, wet air that rushes in when it’s opened, with the goal of minimising the formation of frost. I was totally wrong. It’s a lot simpler, and a great illustration of the ideal gas law.
(The ideal gas law is also known as the “general gas equation”. When I learned about the behaviour of gases in high school physics class, we learned about the earlier Boyle’s law, Charles’s law, and Avogadro’s law, along with some information about those who formulated them. I wondered why we never heard about General Rudolf Gas, responsible for generalising them into a single equation of state.)