As I said, about a million things can cause a sub to sink at depth. Also, the casualty might not have even been part of the torp drill.maybe a high pressure pipe broke in the engine room, or maybe there was a problem and their batteries exploded and breached the pressure hull. A lot of different things could have happened. Submarining is a very dangerous business especially when serving in a third world navy; they’re not known for good maintenance or training.
I served on the Grenadier (SS-525), 1972-73. It’s keel was laid in ‘44, finished in the early ‘50s.
We spent more time in port than out due to repairs. Our various leaks at 200 feet were about 10 gallons a minute. The pumps could handle it.
It was decommissioned in ‘73 and we sold it to the Venezuelans. Apparently they made use of it until the early ‘90s. I was worried about it’s seaworthiness when I was on it!