Shake some supercooled water and you get ice, why?

While in my father's cold garage I noticed a bottle of water that was unfrozen. Knowing that the average temperature in the garage should have averaged well below freezing I was surprised that the water was unfrozen. I picked up the bottle examined it and set it down and went back to work. Moments latter I looked at the bottle and was surprised to find it about 80% frozen (from this fact I should be able to determine its past temperature?).

Now that I think of it I have noticed the same effect with canned soda.

I have searched Google and can't find a link that will explain the physics, why gentle motion is enough to upset an unstable equilibrium.

Can anyone point me to an appropriate link or give a quick explanation?

Thank you for your time.
.

Relevant Pages