This is guesswork
Glass is not, technically, a solid - it is an incredibly viscous liquid. If you measure the thickness of glass in very old windows, it is invariably thicker near the base than the top. So, the bottom will have greater thermal inertia than the top, and is likely to hold it's night time (cold) temperature longer.
So, not only is it colder, than the top of the window and more likely to attract condensation, but the gradient over the surface is likely to cause the air to circulate downwards along the surface of the window, until it reaches it's dewpoint which, if conditions are marginal for condensation, will be part way down the window.
I could, of-course, be completely wrong.
G