[mylife]

if it was not a diesel I would suspect a reactor scram

[mylife]

everything on a sub will kill you, or save you..

[ZOOKER]

They were launching torpedoes, maybe they opened the wrong door in the torpedo tube?
Volatile peroxide fuel and sea water? Explosion hazard
Lead/acid batteries and sea water? Explosion/toxic gasses.

Just to name a few. I would be interested to know whether the hull was actually crushed or not. If the boat was already flooded and pressure equalized when she hit crush depth, the hull would be intact.