The lump was caused by the peculiar chemistry of water. As water is cooled below 39°F, it expands, becoming less dense. This causes water that is at the freezing point to float at the surface, which is where ice forms. The surface starts to freeze at the edges, and the ice develops in towards the center. As this happens, the water below the ice continues to get colder and also continues to expand, forcing water up through the unfrozen center surface area. As that opening gets smaller, the water that expands up through the hole freezes to form a lump. You can sometimes see the same thing in ice cubes which have a bump or even a spike of ice sticking up in the center.