I am inclined to believe the Fire Department and offer a theory as to what happened.
Logic: I have heard of ammonia leaks causing fires.. BUT explosions from ammonia are rare... Hard to make happen.. Propane explosions are easy to do however.
Theory: A small leak in the propane line down-line of the solenoid valve allowed a build up of gas inside the outside fridge compartment.. perhaps a connection a bit loose.. The gas reached the magic concentration... And the burner flame.. at the same time.. KABOOM.