There are a number of reasons why it might have failed, fatigue, deviation from process/protocol but pressure vessels are well understood and not the problem here.
I get that there's a lot of armchair quarterbacking from internet material experts here, but this does seem to be an outlier with this particular submarine.
They can take the pressure. But they're incredibly expensive to check for defects, and the result of a defect is "sub-second catastrophic failure" instead of any warning of imminent structural degradation.