elementary:

http://en.wikipedia.org/wiki/Supercooled

Yes I remember the epic radiator saga :^)
Anyhow, larso has certainly nailed this one down tight.
That wiki link was very interesting, as was the Soo Today story.
As the wiki explanation said, pure water such as obtained through reverse osmosis is required so that there are no seed nuclei for crystal formation to base on.
My guess is that agitating and smacking the container allows a tiny air bubble to act as a seed nucleus and from that point on it is just a runaway propagation thing.
Really "cool" to watch.

Kudos to Larso, for sure!

mownie, just keep wonderin if that phenomenon is in any way related to or involved with the radiator saga (Larso, dunno if you got sucked into that one or not...)