Help me solve a household mystery!

Okay this has my head spinning. I have one of those mini fridges with a teeny tiny freezer. The only thing I currently have in my freezer is an ice tray, 6×2. I filled it up with water about a week ago and stuck it in and forgot about it.

Today I wanted ice and went and got the ice tray. There was ice in all of the cubes except one. Not melty, but nicely, solidly, frozen. The one that wasn’t was not even a tiny bit frozen; it was pure water.

The one that wasn’t frozen wasn’t on an extreme end of the tray. It was 1 from the end, with frozen ice cubes on five sides around it.

My roommate has been away all weekend, so no shenanigans have occurred.

How the heck did this happen?