I have never been able to figure out how people get knocked out instead of being killed and vice versa. I am wild guessing that headshots knock you out and body shots gets you killed immediately. Pretty sure thats not the case but I have no other idea.
You get instantly killed if you are playing solo or are the last person alive on your duo/squad. If you play in a duo/squad you only get knocked out as long as at least one other teammate is alive, because you still have the chance to get revived by them.
If you are knocked out and your remaining teammates also get knocked/killed, you also just die the moment your last teammate goes down.