Re: What made this happen?
This happened in a small window in our conservatory a few years ago, also friends have had the same with a narrow window in their porch this winter. Its quite common apparently. I looked it up on the internet at the time, here's a short description I found just now.
'And here's why double glazed sealed units implode:
This sometimes quite shocking occurrence of double glazed sealed units exploding most often happens in the U.K. between around late October, and early February, but mostly in late December - middle January.
When an inner pane in a sealed unit is the one to suffer breakage, this happens when the inner glass is warmed from the heating in the home, and the outer pane of a double glazed sealed unit is cold from a drop in outside temperatures. When an outer pane is the one to suffer breakage this happens when a cold outer pane is warmed quickly by the Sunlight bearing on it. The pane of glass subjected to the thermal stress cannot cope beyond a certain amount of deflection, and it will suddenly crack along the whole length of the unit, always along the long length, or it can suddenly shatter altogether.'