Why does humidity change suddenly when my fridge's compressor turns on/off?
The frost free feature of the fridge requires the cooling coils to defrost, while the moisture on the coils is changing from solid ice to water and draining into a tray under the compartment the humidity increases, there are several methods employed to do this but that is the reason for the high humidity. The water usually drains into a pan that sits on top of the compressor, the compressor is quite warm when running and the moisture from inside the fridge is evaporated and the next time you open the door the humid air enters and the cycle starts over. I hope this helps on the why but that's what is going on.