Martian

It's very simple. What happened was that he left it in an environment below zero degrees without disturbing it (video's description says the freezer, which does seem to be the most readily available one). The liquid's temperature dropped below zero, but since it was undisturbed no seed crystal formed and it didn't freeze. As soon as he tapped it on the table the liquid was sufficiently disturbed to form a seed crystal and froze almost instantly.

The same effect is responsible for freezing rain.

__________________
I wake up in the morning more tired than before I slept
I get through cryin' and I'm sadder than before I wept
I get through thinkin' now, and the thoughts have left my head
I get through speakin' and I can't remember, not a word that I said

- Ben Harper, Show Me A Little Shame