Kipling, i've experienced that happen many times. Every time you hear that boom a crack is happening somewhere, so my theory is that with the holes in the shack and the bit of weight there it makes the ice more susceptible to crack in that spot. With that amount of ice although unnerving as it is, I don't think theres anything to worry about, good ice is so strong, and its still butted up against eachother, now if it were to crack underneath a truck or something i'd be worried.