Hello gnu! I wonder if your NB spuds had a greater water content? If so, the cold would cause tiny ice particles to rupture the structure of the flesh, making it go to mush. If the US spuds were drier, they would retain their form. I suspect your fridge is quite cold, and the water evaporating from the moister spuds caused the temp to drop futher, to just about freezing, inside each spud. That's my half-pennorth of a theory!