1. Contrary to what I have posted before, clearly the conning tower hatch was deformed by hidrostatic pressure and not as a result of and explosive force within.
2. The portion of the hull around the messroom hatch is not imploded, which means that it was being flooded as the sub sunk.
3. Aft of this, the hull is imploded, so it was dry - pressure-tight bulkhead hatches closed.
4. In spite of having a portion of the aft hull flooded, the stern is intact, proving that the boat hit the bottom bow first, meaning that the greatest flood had happened there. Even if the hull rolled over the bottom, the stern doesn´t appear to have hit it until it came to rest fully on it.
5. If an explosion had taken place in the forward torpedo room (circular running torpedo or, as someone else posted, the shell had hit the bow and not the conning tower due to the boat´s angle when surfacing), could that section have been shorn off?
6. If that was the case, could the angle of impact against the bottom be so steep as to crush the remains of the forward hull inwards and the force of impact so great that the hull fractured near frame #99?

I believe that even if the messroom hatch had been opened by accident, it was not what sank Grunnion, the problem was in the torpedo room.

Again, this is only my guess, and I have seen much better informed opinions than mine here, that´s awesome (and a little intimidating, I must admit)