I've decided that the theory of the marines falling out of ships is now my favorite theory.
"It's raining men, hallelujah!"
Anyway, yeah I agree that my explanations are confusing, and that's because I knew what I was trying to say, but just couldn't really get the message across the way I've been meaning to.
I'm not saying that they died in ground combat, although I do think that's what the writers originally thought when they put them there. All other explanations might be just as plausible. What I guess I'm trying to say is that you can't say my explanation is invalid simply because it violates canon. I know it violates canon, but you're not getting that the writers ignored canon in the first place. Besides, canon is something mostly decided by fans, anyway, not by the writers. They really don't care if they contradict themselves.
Let's do a thought experiment. Let's pretend we were to show that clip of marines to people who have never seen or heard of Freespace before in their lives (not that hard to do), and we asked them which of the following scenarios they think best describes how they think those soldiers died. Do you think they would most likely pick:
A: They fell out of exploding space ships/transports of some sort.
B: They died because they got hit by some sort of bomb evacuating civilians or just "hanging around" not doing much of anything.
C: They died in combat with some force completely unrelated to those bad guys earlier in the cutscene.
D: They died in combat with those same baddies that were kicking everyone's arse earlier in the cutscene.
I think you and I would both agree that they'd probably pick "D." "But that violates canon!!!!!!" So? The creators either didn't know much of or didn't care about canon, so it's likely the writers were thinking "D" as well. Sure, it's possible the other explanations are valid, and hey, they even conform to canon! But, "D" is just as valid even though it violates canon, because the writers probably ignored canon anyway. In fact, it's probably just as valid because
it violates canon.