I think it might have something to do with the convection current of the hot water. Perhaps the higher initial velocity allows the ice crystals to nucleate faster?
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).
Some more ideas. Cold water holds more dissolved air in it than hot water, and the hot water heater would have recently rendered most of those gases out. That dissolved air could negatively impact the thermal conductivity of the water.
Near boiling water is about 4% less dense than near freezing water. If you filled the buckets right to the top, there would be less water in the hot bucket.
