To me the only logical explanation is an implosion.
Because....if they merely lost communication, they could still surface, and I think that they could be found in that case. They could probably inflate air bags that I heard this thing has that might make them more visible.
Anyhow, if they are on the surface, the craft should eventually be found, with the crew alive or dead.
If they were tangled in something, this line of reasoning would require them to ALSO lose communication ability, but it would not be the Titanic they were entangled with because they were not far enough along in their descent to reach it, UNLESS they lost communication and still decided to proceed.
All other options require TWO different things to go wrong at the same time, which is less likely. And it would have to be communication FIRST.
An implosion makes sense. I learned on the news last night that these things cannot make descents indefinitely - apparently some kind of material fatigue sets in.
There would likely be no leak, no warning, just BOOM. The pressure is too immense, unceasing, it never lets up for a microsecond, once there was the slightest weakness, the tiniest breach or bend, it would seek out the next weakness instantaneously for all intensive purposes, the chain reaction would be insanely fast.
Think of it as every square inch of the thing constantly having the force of a bullet on it every second.
I hope I am wrong, because of the deep location, total darkness, it is a scary death to consider.