just finally made time to watch the last 5 videos or so (including the one posted after this), great stuff
at first I thought this "hands up" animation was just the falling animation, but I don't remember that having a waving arms part to it, and I think the arms were held up higher in that, but it's worth comparing them
now I suspect the animation was intended for marines and other allies if you started shooting them, to signify they're not a threat
somewhere along the line I'm guessing they decided against it so the ties got cut between the human allies and that animation, but since Flood allies were never accounted for (because they only show up in one place and you're never supposed to interact with them), they're the last link to this animation
it could also just be a buggy version of some other animation, which honestly seems like the most likely answer, but that's boring so let's go with the other one