OK, how is this for a guess BH:
When aircraft is flying in rain/cloud etc. water collected around the potable water access hatch and froze, lump of ice gets bigger and bigger until it falls off and falls through someones roof/head etc. also, on arrival at destination access hatch may be frozen solid and turnround time considerably extended whilst ice is chipped off/melts.
Solution: Put in a rain gutter that directs water away from access hatch and prevents ice forming?