In an interview with a submarine captain posted a few pages back mentioned even if the submarine somehow was caught on something it wouldn't explain why their communications would be down.
I'm leaning toward it imploding. Although the sub is supposedly safe to the depth of 4000 meters, and the Titanic wreckage is at 3800 meters.
And others have mentioned this 'tourist expedition' has been performed eight times, to a few dozen times, previously in this same sub.... so many things could have still gone wrong.
I'm a trained metallurgist, mostly related to welding, and I can testify that micro-cracks can form and be impossible to detect with the naked eye.
The Titanic wreck's depth is 3800 meters, and the water pressure is measured as '400 atmospheres.' I vaguely recall from the late 90s that's 2400 pounds per cubic inch.
That amount of pressure could cause a micro-crack to expand rapidly. The scary aspect about manned deep sea submarines is there's no way microcracks can be detected before 'the plunge' into the depths.