A question:?

[Rhodes]

A question about this:

Ok, so, a few months ago National Graphic channel have a program about ww2 subs. It was called WW2 hell Under Water - http://www.natgeotv.com/za/shows/nat...-under-the-sea
with reconstitutions and etc.
With stories from U-boats to Fleet Submarines, their commander and their most great patrols.
Well, do to on site filming, it is good to see Otto Kretschmer in a U-boat Turm with radar, Naxos warning devices and so on - ok, simple they film those scenes on U-995. But it is besides the point.
My question is about the episode regarding Hetherington O'Kane and his submarine USS Tang.
During the episode explaining the last patrol of USS Tang, the narrator said that they encountered a storm when travelling to the objective area.
What I can saw, from the reconstitution in the program , the fleet sub was travelling at surface, no one at surface and bouncing from and side to the other, when the inclinometer reach 70º degrees, O'Kane character asks what happens if the sub could not return to strait position and got a reply like:"some times it can't return". But the sub returns to his normal position.
Then narrator came in to action and explain that if the submarine deicide to submerge during the storm, if would sunk! That is my question, why?
When seeing the episode I though, well, why not submerged and go calmly, under the water and storm, etc. And got that sentence. ?
I understand trying to submerge with a 70º of inclination, but my doubt is more when the sub is in a normal roll position.