Vortex

I don't know about this effect on lakes, but on precipitation, this is called "supercooled water droplets"; it's pretty much the same thing happening here — water drops up in a warm layer of air clump together into a rain drop and fall, and they fall through a colder layer of air above ground but below the warm layer. When they do that, their temperatures drop below freezing, but they stay a liquid until they hit the ground, when they spread out and instantly become über-tenacious, hard ice. That's how ice storms happen.

epepke

Supercooling.

I doubt the story about the lake, though. A lake has a lot of sites for heterogeneous nucleation. Supercooling usually happens in a smooth container.

Autonemesis

I've also seen where you can heat pure water in a glass container to a few degrees above boiling, without it coming to a boil. But as soon as a metal object touches the water, it boils in a flash, almost an explosion. The reason is that boiling in a pot on the stove begins in the microscopic pits and cracks in the metal. But a glass container has no pits or cracks, and so the water can continue to rise in temperature past the boiling point. When you introduce the metal object, nucleate boiling begins on its surface and spreads rapidly.