Ice Worms! Yep. Ice worms did it. They burrow through the ice making it very porous and weak, and reproduce very quickly, multiplying by the billions in a single day on an exponential logarithm. They are microscopic invertebrates like amoebas and are common in glacial ice and the polar caps on Mars. They likely arrived and invaded this pond by shear chance, with the dust remants of an ice comet or a meteor like the one that exploded over Russia earlier. Once they have devoured the ice, it opens into a large hole in the ice, often circular but can also create ellipses and polygons. Some have even been suspected to have intelligence since they create tic-tac-toe games in the ice. This is just something they do for fun.