It is quite common, particularly when the wine cooler or wine cabinet is first switched on, to see the formation of frost/ice on the back wall of the unit.
When you first receive the unit, the door will most likely be open quite often as you load new bottles or wine, remove packaging or re-arrange your shelving. By doing this, it may be that moist air from outside of the cabinet is entering the wine cooler and raising the internal humidity.
A wine cooler will maintain the internal humidity between 50 and 80% and has sensors to ensure excessive levels aren’t reached. If moist air enters the wine cooler, during the cooling period, the water molecules will condense and potentially freeze on the back wall of the wine cabinet. This is were the evaporator is generally were the evaporator is located and thus the coldest section of the cooling system.
Don’t worry, during the period where the cooling system is off, the ice will melt and in most wine coolers there is a purpose-built drip tray at the bottom of the cabinet. The drip tray leads to a small water container that is located next to or on top of the compressor which is the warmest component in the system. From there, the excess water will evaporate and the humidity level will return to normal levels within the cabinet of between 50 and 80%, this is in effect the systems self-regulating humidity control as frost/ice build up will only occur when relative humidity levels are above 90%.
Try not to open the door of the wine cabinet too often as the environment is being managed within the wine cooler.
If frost/ice build up persists, contact the manufacturer for expert advice and ensure you have checked the door seal as ice build up can restrict the door from closing properly which will mean more moist air is entering the interior.