So why do sealed units fail

Temperature:
Sealed units are normally exposed to high temperatures in the Summer months and freezing cold conditions in the Winter, the external temperature has an impact on the double glazing.

The high Summer temperatures causes the air trapped between the two panes of glass to expand causing the centre of the unit to swell like a balloon, as the temperature drops the air contracts and the unit goes into negative deflection causing the glass to contract in the middle.
This constant expansion and contraction is called solar pumping, the stress this causes on the perimeter seal eventually causes the seal to fail.

When the seal has failed moisture is drawn into the unit when the temperature drops, and is expelled from the unit as the temperature rises, this daily transfer of air quickly saturates the desiccant crystals inside the spacer bars and moisture condensing on the glass is inevitable.

Solar pumping is the main cause of sealed unit failure and is prolific on south facing elevations where the units are exposed to high temperatures.