I wonder what they were doing when it happened?
After initial manufacture, or again after completing major repairs to a fuselage structure, it is the practice to carry out an overblow check. The fuselage is pressurised to at least 150% of the rated Maximum Differential Pressure. For passenger transport aircraft the max diff is in the order of 8.6. p.s.i so the overblow would be to 12.9 p.s.i. The purpose of this operation isn't to see if the fuselage can withstand the pressure, it is done to work harden the structure and improve the fatigue life of the whole pressurised fuselage.
In the case of this KC135, either there was a pre-existing major structural fault or the pressure limit was exceeded by a much wider margin than 150%.