“A failure in engineering.”
My understanding is that a welder brazed a critical joint, rather than welded, which failed under pressure.
Many years ago I met one of the principal investigators of this accident. According to him, the initial failure was a braised joint near to a hull penetration, which allowed seawater into an electrical control room. The failure caused a cascade of events which the crew could not handle.
In those days, there was no reliable way to test a braised joint; as a result of this accident, there is now a procedure which ultrasonically verifies the quality of the joint.
https://en.wikipedia.org/wiki/USS_Thresher_(SSN-593)
“Deep-sea photography, recovered artifacts, and an evaluation of her design and operational history permitted a Court of Inquiry to conclude Thresher had probably suffered the failure of a salt-water piping system joint which relied heavily on silver brazing instead of welding; earlier tests using ultrasound equipment found potential problems with about 14% of the tested brazed joints,[12] most of which were determined not to pose a risk significant enough to require a repair. “