Are there any physicists or chemists out there who can help me with this dilemma?

When I put a water bottle overnight in the bottom of my freezer, the water does not freeze. When I pull it out and set it on a counter, it immediately begins to freeze over and form a very fine slush. I am not complaining one bit. In fact I enjoy drinking slushy water very much. It's like a flavorless Slurpee from 7-Eleven. I'm just curious to how this phenomenon occurs.