This is caused by condensation. The same thing that happens to a cold beverage on a hot day. The cold glass collects the humidity from the air inside the home and it will build up on the bottom of the glass pane where it’s the coldest. This is a major problem on old windows that are not thermally efficient. This can also happen on new efficient windows when temperatures outside are very cold and the humidity inside is very high. It’s recommended to have inside humidity around 15% in the winter to prevent this.