It probably has something to do with warm air inside the house meeting cold air outside the house at the windows. (You may have noticed that it happens more during cold weather, when you're more likely to have the heating on). As far as I'm aware, the only way to stop it would be to have the windows open all the time, which sort of defeats the purpose of switching on the heating. Best thing to do is open the windows first thing in the morning to allow them to dry off.