When you open the freezer door and let everything thaw out, the temperature inside the freezer will start to rise, causing the frozen items to melt and return to their liquid state. This happens due to the process of heat transfer.
The cause of thawing is primarily due to the movement of heat from the surrounding environment into the freezer. When you open the door, warm air from outside the freezer enters the cold environment inside. The air inside the freezer is at a lower temperature than the air outside, and according to the laws of thermodynamics, heat will naturally flow from the warmer air (outside) to the colder air (inside).
As warm air enters the freezer, it comes into contact with the frozen items, such as the ice cubes, frozen food, or any other frozen substances. Heat from the warm air is then transferred to these frozen items. The heat energy raises the temperature of the frozen items, causing them to melt and turn back into liquid water.
The process of thawing will continue until the temperature inside the freezer and the frozen items reaches a point where they are in thermal equilibrium with the outside air. At this point, the melting will stop, and the items will remain in their liquid state until they are either used, refrozen, or allowed to evaporate.
To prevent unnecessary thawing, it's essential to keep the freezer door closed as much as possible and avoid frequent and prolonged openings. This will help maintain the low temperature inside the freezer and keep the frozen items frozen for a more extended period.