I have been using Second Chance (PRO) since I started playing Black Ops the day it came out. All of my friends also use so that we may work as a team and revive each other. We have all noticed that sometimes when we get downed, we mysteriously die almost immediately.
The amount of time you survive while downed depends on whether you have the Pro version of the perk unlocked or not. Without Pro, you die in about 5 seconds of being downed with no possibility of being revived. With Pro, you can survive for about 30 seconds and team mates may revive you. Either way, sometimes you suddenly die more quickly then this. I have been downed before and dies before I can get out the words to ask my team mates for a revive.
We originally thought that it happened when you tried to descend a staircase while downed. This is not true though because I have actually crawled down entire stair cases several times while downed. I have also mysteriously, quickly died while on seemingly flat surfaces.
I can also assure you that I am not accidentally holding the suicide button (X on my XBOX).
Does anyone know the conditions for this premature death so that we may avoid it? I hate getting downed, killing my attacker and then dying right away, giving my attacker a kill he did not earn or deserve. Plus I loose my kill streaks for no reason.
EDIT: I can also state that it is not something killing me that I didn't notice (like stray bullets or random flashes). This oddity happened even before the update where the original person to down you always gets the kill. And they would be the one credited for my mysterious, premature deaths even if I had already killed them earlier.