What could cause a toilet to crack in winter when the plumbing has been drained and any winter use coincides with filling the bowl with plumbing antifreeze?

There may be a manufacturing flaw in the tank, as materials used in bathroom fittings are not susceptible to cracking in the cold. In casting vitreous china, the occasional bubble can be trapped. If a water-filled bubble freezes during winter, it could expand enough to crack the tank. It can also be tricky to completely drain all the bits and pieces of the mechanism that empties and fills the tank. It’s possible that, though a tank looks dry and empty, there’s enough water left in the valve assembly to freeze and damage it.