Freeze, thaw, freeze, freeze thaw.
Your outside pots are having it hard at the moment. - expanding and contracting on a daily basis is fine for some materials, but for terracotta, and some plastics the constant pressure put on them at this time of year is too much. Cracks start to appear, and then water gets into the cracks, - expands when it next freezes and your pot falls apart.
Here are my tips for keeping your pots in one piece this winter.
1) Ensure that drainage holes are as large as possible and are clear of roots and debris
2) Stand your pots on feet or bricks so that water can drain out
3) only water your pots in the morning when there is no frost, - so that water can drain through before it freezes again at night.
4) move pots to the shelter of house walls if possible, to raise the temperature slightly,
5) wrap in fleece or bubble wrap to help keep temperatures more even.
If your pots do fall apart this winter, - use the broken pieces for drainage crocks, and make sure that the pots you buy next year have larger drainage holes.