Immediate implosion if a far more likely explanation because give that we knew it imploded the most likely time would be immediately after the pressure vessel reached maximum or near maximum stress and it failed. The time actually is not the biggest factor; it is the number of cycles the pressure vessel has endured. An implosion after a relatively low number of cycles is consistent with past incidents with pressurization failures.

As was pointed out, the submersible is pretty large and its a essentially a large cylinder which induces stress in the middle. Usually you make deep sea submersibles out of a sphere or spheroid because you don't have this failure modes.

It isn't surprising it failed. They were strong claims that the company did do enough test, rejected safety advice and even had the CEO claim that at one point the titan wouldn't be able to reach the titanic because it (a model maybe?) had experience a pressurization failure. To say nothing of the materials they were using.

Just comically bizarre that anyone involved in the project thought it would somehow survive let alone be safe.