What causes window condensation?

When the seal on a thermal pane window fails, the trapped air between the panes is pumped out as the air space heats up and expands. Humid air is drawn into the enclosed space as the window cools down.

This heating and cooling cycle occurs each time sunlight hits the window. Over time the humidity level between panes becomes high enough that water vapour will condense on the glass. Think of the outside of a cold beverage glass on a hot summer day.