I noticed mushrooms freeze in my fridge when nothing else does. Can anybody explain the phenomenon?
Your fridge is infact below freezing temperature, perhaps just slightly. Water (the main liquid in mushrooms) never freezes at temperatures above freezing (0C/32F).
Mushrooms have a lot of surface area and are very light so their temperature can match the fridge easier. Try placing dry lettuce there and you'll notice the same thing.
If it's a small and not so full fridge, you can raise the temperature a bit.