Got to be cavitation generated along the sides due to the shock.
WHERE”S CLIFFY!? He had a great one on why you feel smarter when ya drink beer. :-)
“Got to be cavitation generated along the sides due to the shock.”
I suspect you are correct. The initial shock would cause bubbles to form due to the pressure wave. These bubbles would then collapse i.e. cavitation, and form new shock waves that would then spread into the main mass of the beer. It would be a self perpetuating cycle until most of the beer is foam. In essence you have an unstable liquid CO2 mixture that just needs a shock wave to start a rapid change to equilibrium.