Submersible Implosion - fiber-composite issue
The main weakness in fiber composites is that they do not do well in compression along the fibers. Any load or condition sufficient to buckle (severely bend) the fibers can lead to failure. This is much more the case with "brittle" fibers such as Kevlar or carbon - Nylon, polyester, etc. tolerate this better. When these brittle fibers bend sharply, they lose much of their strength, and when a large enough % of the fibers are damaged, the composite can fail.
This phenomenon was evident in analyzing air spring failures when I worked at Bridgestone/Firestone. That's why tires that have been run severely under-inflated totally come apart and the tread section can completely come off. This also applies in racing to CF tubs or suspension components, and that's why they can eventually lose stiffness or break.
So, IMO, too many severe compression cycles on the sub primed it for the resulting implosion failure.
Comments from James Cameron, deep sea explorer
Some experts, however, weren't surprised — including film director and deep-sea explorer James Cameron.
Cameron told ABC News that he believes the Titan's hull began to crack under pressure and that its inside sensors gave the passengers a warning to that effect.
"We understand from inside the community that they had dropped their ascent (I think he meant descent) weights and they were coming up, trying to manage an emergency," he said.