My thinking/thoughts. Elevated tanks use gravity to supply water. I do not know of any that would be pressurized. Tanks have vents to allow air to escape as the tank fills. Tanks have overflow ports should pumps not shut off via the sensor placed at the desired “full” mark.
Sorry my last post I was kind of thinking out loud. This tank was not elevated. It could be that there was enough corrosion at the base of the tank that the bottom failed while it was still at normal water distribution pressures. Cutting in the tie in that they were working on might have found a weak spot. No explosion necessary.