>Are there no pressure safety valves?
THERE is the operative question.
Any water tank of that kind would be open to atmosphere and at atmospheric pressure.
It could be slightly pressurized by overfilling the tank but that would be immediately obvious because water would be shooting out the vent and splashing on the ground.
The only thing that makes sense to me is that the head space inside the tank was somehow field with an explosive mixture of gas that was ignited by the welder.
Because water is incompressible the extreme pressure caused by the ignited gas would cause the weld at the base of the tank to fail. The pressure of the explosion and the weight of the water lifted the tank.
My guess is that natural gas somehow leaked into the tank either from the ground or from some connection to the incoming piping.