I would go with excess CO2 being drawn in through the skimmer. This time of year its cold so all the windows and doors are shut tight so CO2 builds up in the house, the skimmer does a great job of incorporating that CO2 into your water. Since CO2 depresses PH voila.
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~David~
300+gal multitank, seagrass, mixed reef, clown anemone, refugium, and sump. | 
