What is the internal temperature of an object like this when it first enters the atmosphere? The heat of entry would heat the surface rapidly and the result is tremendous thermal stress between the hot exterior and relatively cold interior. How hot do you suppose the surface of this thing this thing was when it hit the lake? It appears this is a stony - as in non-metallic - meteorite. Depending on the material configuration of this mass, when it became immersed in the freezing lake water the rapid cooling could cause it to fracture or crumble easily, much like putting a hot ceramic straight from the oven into cold water or pouring cold water on hot rocks surrounding a campfire.