Keep in mind that the gauge is 4 feet down, now under the ice. I'm sure that once the lake completely freezes over, as it has now, the environment under the ice can be very different from that above.
If you read this interesting description of lake flip from the Squam site, I suspect it might have something to do with temperature layer changes in the water under the lake.
http://www.squamlakes.org/sla/ecology.htm