Uh-oh! You slipped next to an acid pool, or blew up a barrel at close range, or fell just a bit too far, or mistook a steam jet for a sauna, or just weren't fast enough on that hot grate, or stood too close when blowing that secret wall, or . . .
You either met up with some alien handlers, or else some other armed human caught you with your armor off and your health down. Except for those grenade-launching alien handlers, the humans are all pretty slow and won't do you much damage before you can take them out.
The last thing you saw was an agitated chestburster or a very determined facehugger.
You were successfully downed by a warrior. This is definitely the leading cause of mortality in this game. Believe me, you have lots of company.
You stumbled onto a pack of young or adult dog aliens and didn't survive the encounter. If they were adult dogs, you died honorably.
Death by queen. Without question the most stunning exit possible.