Question about Damage and Torps

[Pevs86]

Gotta first explain the situation...

Sitting in between the middle and 2nd line of a Taskforce, Heavy Cruisers behind me and 2 ACs infront of me.

I fire 4 torps at the 2 ACs, and as I'm lining up the Cruiser, there's a MASSIVE explosion diretly over (or maybe inside?) my Aft torp room.

I checked the log, 3 of my 4 torps hit the ACs. Checking the log it didn't miss, so I'm assuming it didn't run, detonated inside the sub? But then it was my rear tubes and engine room that took all the damage.

Enter problem number 2... huzzah!

I manage to control flooding at about 160m, by then I'm basically vertical so blow ballast and come to the surfice and level out - result - immediatly come under fire, of course, but no hits - just some AA fire - so I set periscope depth, as I said - flooding controlled, boat level. Now after about 15seconds at PD I begin to sink like a brick, all the way down to death depth

So the way I see it is,

The massive explosion could only have been:

1. The H Cruiser launching a massive, single, DC

2. The stray 4th Torp firing, sitting stationary in the water (whilst I moved forward) then detonating whilst I was over/underneath it.

3. Detonation in the tube for whatever reason.

The sinking I don't understand. Damage reports said flooding was fine, the only areas that were damaged were Engines and Aft Torps, all pumps were intact.

If anyone can shed any light on anything I'd be grateful... was really impressed with saving the sub only to have glory stolen at the last moment not to mention my first 2 AC kills...