At over 10 kilometers up in the sky its rather cold at below minus 50 degrees centigrade.
If some moisture collected between the double panes of aircraft window glass at lower altitudes, at this altitude it freezes.
Two types of freezing occur.
Droplets of water that had already formed at lower altitudes simply freeze like those on the left. Water vapor that freezes skipping the liquid phase and going straight from gas to solid form the spiky crystals on the right.
The crystals form most easily on other crystals or where the glass surface is scratched or otherwise damaged as can be seen in the photo below.
The frozen droplets at slight magnification with the sun hitting them at just the right angle produce some beautiful photographic results.