Please forgive me hypothesizing about an awful tragedy, but I can't help being reminded of the 'Ouzo' accident; in that position they were close to the shipping lane and indeed a submarine exercise zone.
I suppose with the boats' chequered history there may have been a reluctance to put a call out, but the only way I can really imagine her going so quickly would be a collision...
Or getting their gear caught by, erm, something moving underwater, and being dragged down?