the breach is because the hermetically seal unit as failed.
when the rubber gaskets that you see around the glass loose there suppleness they go hard,this allows water to run down between the glass and the gasket......but this water should escape from the bottom of the frame through drainage holes,these can either be on the front of the frame,lets say above the sill or sometimes in the bottom of the frame and will run out on to the sill
on opening windows if you open one and look up you should see these drainage holes
One way of knowing that the gaskets have failed is the tell tale signs of black run marks on the main frame were the opening window will close against,you can also test this by spraying water on the opening window(when open) and watch the water escape from the drainage holes
When the hermetic seal has failed then water,be it rain,condensation or WFP water will evaporate through the seal and into the two panes of glass