I had an ASW TF torpedoe a JP sub in the tutorial. It listed the torp as cause of death in the sunk ships list, and the Combat report said it as well. I am guessing that they caught them on the surface and BOOM! The sub did sink quickly with no depth chargeing.
Maybe[;)]the sub torpedoed itself. There was at least one instance that I know of, when a US sub operating IIRC in the area around Formosa, let off a spread of torpedoes, only to have one turn back and hit the sub, with wholly predictable results. IIRC only the people on the bridge survived. I am sure some forum grog will have the actual info.