Question about fire on a sub

[frau kaleun]

Actually this is a question about something that happened in the movie "Below," where some dudes got burned up. So it's sort of a technical question about something that might (?) happen on a sub.

If you have not seen the movie it's about a US sub in the Atlantic during WWII (supposedly they used a Gato-class for filming if that matters), everything else is irrelevant except for the fact that at one point they have been submerged for a very long time. So they are getting low on oxygen and worried about not just the CO2 content in the air but judging by the dialogue and the gauges they keep looking at, the hydrogen content as well?

Anyhoo during this time the rudder controls become inexplicably stuck and in their attempts to force the issue they completely destroy the big wheel or whatever that they would normally use to steer the boat. So the chief says they will access it by cutting into the line somewhere aft, in proximity to some other things that raise concern about the possibility (I assume) of fire breaking out due to sparks (?) but the chief says it will be a "hydraulic" job with no welding, or something to that effect.

The chief and most of the remaining crew go aft and all the hatches are sealed as a precaution while he and a couple guys get to work. They cut the line and are, I presume, fixing the problem when all the lights go out. Somebody says he knows why (I can't remember the reason) and goes to fix whatever that is. Meanwhile someone in the next compartment opens the separating hatch and starts complaining about them being in the dark and the chief yells at him to close the hatch. It cuts to a scene, I think, of some wires or something starting to glow, presumably because the guy who went to get the lights back on reconnected something or flipped a switch.

Then it cuts to the control room where the officers are talking and the lights flicker. Then they start trying to raise the crew who went aft on the "intercom" but nobody answers. They go check the control room hatch and it's warm. So they open it and go back to find everyone aft all burned up.

So here's my question: I get that when the one guy did whatever he did to get the lights back on, it heated something up or threw a spark and there was some kind of flash fire aft that killed everybody back there, obviously it burned hot and fast enough to do that and then just die out because nothing appears to still be burning when the officers go back to check it out. At first I thought that the guy who opened the hatch caused it by opening the hatch, but it's not like there was anything different in the air in any other part of the boat, it was all "bad" so that doesn't seem to be relevant except that it put more of the boat at risk if there was a fire. Anyway, what exactly happened? Was it the fact that the hydrogen content was so high that started the fire? Was it the combination of that in close proximity to whatever it was they were worried about in the location where they were working (I think they were close to the batteries, I'm not sure)?

I didn't realize that hydrogen was an issue on submarines of that era, I've never heard it mentioned with u-boats - only oxygen vs. CO2. If nothing else watching that movie has made me more interested in getting to know the "insides" of the US subs, because I don't have the layout in my head and that made it very disconcerting watching the movie. I kinda felt the way I did when I first watched Das Boot and could tell by the context that something happening was bad, but didn't know all the technical details of why.