It's just a normal physical reaction (condensation) that occurs whenever a cold object is introduced into warm, moist air. The moisture in the air is cooled by the object and the droplets form on the cold object--just like a glass of iced tea on a hot, humid summer day.
It means that the air outside the refrigerator is more humid than it has been.
The condensation will dry in time, as the cake comes to room temp. It's important not to touch the cake until it is dry.
To prevent it, box the cake and wrap the box with several layers of saran before putting it in the fridge. Take it out and allow it to come to room temp for several hours before unwrapping it. That will eliminate the condensation being on the cake.
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I heard that some decorators place a plastic bag over the cake before taking it out of the fridge and condensation forms on the outside of the plastic bag. Have to try that some day. Was always afraid the sticky fondant would stick to the plastic bag but I think you remove it after a few minutes.