Why do mushrooms freeze in fridge at temps above freezing?
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.