Hot water melts snow, and the ground is much colder than the snow which isolates heat. So in your case the simple explanation is just that, the hot buckets melted the snow/ice and therefore stopped being isolated against the cold ground so froze faster.
The assumption being they were placed on snow, which I would argue is an unlikely assumption to hold up. We have much more days with freezing temperatures than days with snow lying around and even then with lots of snow there are enough places that don't have any (like a balcony).
