With the changing of the seasons, we normally turn on our home heating systems. Many times this is a dry heat. Air will seek a normal "saturation" point. This means that air of a certain temperature will hold so much water. If you have recently turn up th heat in your home the temp rises, heat usually removes moisture from the air, and replaces that humidity with available moisture--your aquarium water.
Someone has already mentioned that water that is being aggitated evaporates quicker. This is true because the surface tension is broken making evaporation easier. This can be lessened by making sure that your aquarium's lid is tight fitting.
So, to make a long story short, just grab a bucket and fill the tank back up.