At the same time, the fuselage was bent downward at the extreme front and aft ends with millions of pounds of force. And the interior of the plane was pressurized beyond normal levels to about 10 pounds per square inch — not typically a requirement for this test, but something Boeing chose to do.
In other words, Boeing had previously done the 1.5x pressurization test - and passed. Then, although it's not a regulatory requirement, they
again pressurized the fuselage to 1.5x when they did the wing ultimate load test - when it failed at 99% of target.
I believe this is where the theories come from...overpressure the hull to prevent buckling...