This happens when the dehumidifier is in the automatic defrost mode. When moisture from the humid air collects on the cooling coils it can sometimes collect too rapidly and start to freeze. After the unit reaches target humidity the fan will continue pulling in the room temperature air to help defrost the coils.
You can usually tell if this is the case when the air exhaust is emitting cool air instead of warm air.
This tends to happen when the room temperature starts approaching 65 Degree Fahrenheit, but your unit can still operate to temperatures around 40 degree Fahrenheit.