Why does stirring liquid water in a freezer tray make it freeze instantly?

[ax0]

I'm sure this is a totally amateur question, but today I got one of those big ice cube trays for 2x2x2 inch cubes. I filled it up and put it in my freezer, and about 6 hours later checked on it but the water was not frozen at all. Not even a crust on top. I put my finger in one of the squares and swirled it and in that moment the purely liquid water became a snow-like consistency. I did it again with the other squares and the same thing happened. What causes this?

 

[davenn]

I put my finger in one of the squares and swirled it and in that moment the purely liquid water became a snow-like consistency. I did it again with the other squares and the same thing happened. What causes this?

a very poorly operating freezer ... my cubes would be solid ice after 2 - 3 hours
a 1.25 bottle of coke will freeze for me in a couple of hours

 

[Drakkith]

I put my finger in one of the squares and swirled it and in that moment the purely liquid water became a snow-like consistency. I did it again with the other squares and the same thing happened. What causes this?

Sounds like the water was in a supercooled state. https://en.wikipedia.org/wiki/Supercooling

 

[davenn]

Not even a crust on top. I put my finger in one of the squares and swirled it and in that moment the purely liquid water became a snow-like consistency.

Sounds like the water was in a supercooled state. https://en.wikipedia.org/wiki/Supercooling

yes and the adding of the finger to stir it added the seeding process for it to freeze
reasonably pure water can be lowered below its freezing point without freezing ... till seeding occurs