My hypothesis:

So, basic principles out of the way first, dark absorbs more sun, white reflects it. As the snow melts and reveals the darker colored water beneath, this will begin a runaway feedback loop that will slowly melt more and more ice. Assuming it’s not too cold out, anyway.

Since this is actually a runaway feedback loop that is going to eventually melt the whole surface of this body of water, we just need to get it started, and everywhere it starts, it’ll spread from. All we need, is something that darkens the surface of the snow.

In the case of that center circle, it’s hard to make out, but I think I see a stick jutting out in the exact center. A brown stick, no less.