A seal is applied to fix the spacer bar to the glass prior to the application of a perimeter seal to create a hermetically sealed air space.
The weather is the main reason for a sealed unit to fail and you will probably find the units on the south facing side of your property will fail quicker.
When the units are exposed to High/Warm temperatures in the summer the air inside the unit expands and in the freezing cold winter months it contracts.
The stress this causes on the perimeter seal eventually causes the seal to break away from the glass (to fail).Once the seal has failed, moisture is drawn into the unit when the temperature drops and is expelled from the unit when the temperature rises. This eventually saturates the desiccant crystals inside the spacer bars and moisture condensing on the glass is visible.
Moisture/Humidity trapped in the unit turns into steam that rises to the top of the unit, as it gets cooler the steam condenses on the cooler glass surface (outer pane) and runs down the glass.