The naval liaison accompanying the expedition for the documentary postulated that the reason for the loss of the entire crew was, first, the severe loss to the command-and-control personnel on the bridge and gunnery control, as well as that of the crew itself .. approaching 80% casualties .. in the engagement with the Kormoran and, second, the torpedo hit forward.
Because of the severe loss to the crew and command group, the Sydney was limping towards land at a reasonable speed while the remainder of the crew was going about their damage control duties, probably mostly belowdecks and fire-fighting above. Because of the extensive damage to the bow .. the torpedo supposed struck between A and B turrets .. and the speed of the vessel, the bow probably wasn't hanging on by much.
The bow was found some distance from the rest of the Sydney, so it was assumed that it broke off and sank suddenly and unexpectedly, causing the remainder of the ship to be driven under the water by the forward speed of her engines. Those belowdecks wouldn't have had a chance and those above would have been caught by surprise and left adrift without rafts or boats. Being quite some distance and over the horizon from the Kormoran, who was also in the process of sinking and being scuttled herself, those few sailors who were alive in the water would have probably perished fairly quickly.
The underwater pictures of the Sydney are phenomenal, as are those of the Kormoran. In one picture, it quite clearly shows a hole in one of the turrets, created by a German armor-piercing shell, just below and squarely between two of the guns.
Quite a story.
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RESISTOR
Swear allegiance to the flag, whatever flag they offer;
Never hint at what you really feel.
Teach the children quietly for, someday, sons and daughters
Will rise up and fight while we stood still.
Der Elite Møøsënspåånkængrüppen ØberKømmändø (EMØØK)