After a little bit of research, I found that the lanes are indeed due to the lake ice melting from the underside first. That's the part I got right. But it turns out that the melting-from-below happens because of heat convection in the waters below, not from currents flowing and eroding the underside of the ice. Once the snow on top of the lake either melts or blows off, the ice sheet is transparent enough to conduct sunlight into the water below and warm it up.
I love finding this stuff out!