Water freezing in the drum internals, I'd guess.

One reason to not set your parking brake in freezing weather. The shoes may not release fully and you can end up with some really, really hot parts.

When it warms back up, you should probably also bleed the brakes completely, starting with the lines that are longest (rear, front passenger, then front driver) with fresh, never-opened brake fluid. Check your master cylinder cap to make sure it's sealing well, too.

Brake fluid also really likes water, so never allow the system to be open for long, don't use old, opened fluid cans, etc.

John B.