Could have been any part of the system, although carbon fiber is especially problematic... failure mode is to shatter with no warning, only regular NDT using imaging tech could have reliably seen the signs coming although this application is so extreme who knows. Metal is more predictable and will tend to show signs in advance which you can inspect for.

Rumor in submarine circles is that the front window was a commercially available submarine window but not rated for anything like the depth in question.

The USN DSV Alvin is a good example of how to do this right, IIRC it's pressure hull is titanium and it's made thousands of deep dives over many decdes. It was used to survey the Titanic initially, although that whole operation was a smokescreen for a classified inspection of both sunken US nuclear attack subs... so vastly different cost constraints with Uncle footing the bill. Similar to Glomar Explorer.