What most people do not understand in the dynamics of a thermal eruption, in this case water, water expands X 1760 when going from a liquid to a vapor,
Also the temperature a material goes from "liquid" or "solid state" to "vapor" changes with pressure, why water boils at a lower temp. at the top of Pikes Peak than at sea level.
So the perfect storm, the water heats up to far above the normal boiling point because of the pressure, then the "vessel" fails, at that exact moment as the pressure is "relieved" the water expands 1760 times it original volume and that is what causes the walls and roof of a house to be blown out, if a vessel of the same size was to fail under the same original pressure when filled with air it would be not nearly as destructive.
And if filled with cold water, like when we did "hydro testing" where I used to work very little damage would occur as as soon as there was a failure the pressure would instantly drop to zero as you cannot compress a liquid.
This is why Mt. ST. Helen was SO destructive, same thing happened but with molten earth...