Excuse me if I'm talking through my hat but.
A few years ago there was a lot of attention paid to aging aircraft issues coinciding with the spectacular 737 incident in Hawaii and also some things happening to 747s.
One comment I read was that the 747 was particularly susceptible to stress and fatigue because the cross-section, particularly at the nose end, departed so much from the circular shape which internal pressurisation would try and force the fuselage into.