ICE!
The water is absorbed into the clay pottery/ceramic and freezes... and expands when it does, breaking the structure of the pottery from within the walls itself. Using a plastic pot will stop this from occurring,as the walls of the plastic pot don't absorb any water..... however keep in mind that if the new pot holds water (no drainage), or has media that holds a lot of water, this too can expand when it freezes out and break the plastic pot also. (Not nearly as often or as bad, but I have had it occur, especially when the pot doesn't have drainage.)
Hope that answers your question. I have had that happen for years during the winter. If you even store ceramic/clay pots outdoors, they will tend to break up, sometimes even when stored upside-down. It all depends if they have time to dry enough before freezing weather, to not have the water (in the walls of the pot) freeze and expand. A little water seems to do no harm, as I do overwinter pots outside sometimes, but if you care about them, take them inside!
The broken pottery is excellent for covering the holes in the bottom of the planting pots, so don't let them go to waste! (A nurseryman taught me to use broken pots for this back when I was a kid, and it is the BEST!)
Good luck!
Indeed the roots could cause breakage, but ice is the likely cause. Plastic pots usually split from root growth a lot easier than clay pots.... as they are flimsy and the sun makes the plastic brittle in time. Not that it can't happen with clay pots, but it has been a rare occurrence from my experience. Mentioning that it froze here is the key, as breakage is VERY common with wet clay pots.
