OK. Son woke up (finally).
It has to do with "super chilling" and pressure differentials.
That is, the temperature of the liquid is below freezing, but it is static so hasn't frozen yet. opening it changes the pressure and/or surface tension so it freezes. At just the right temperature, you could make it solid just by shaking it.
Kind of like the opposite effect of super heating your coffee in the microwave and it explodes when you put a spoon in it, disrupting the surface tension.
Phase diagram for water:
