It rejects its own bones and steals yours away.
Also, this happens in other games from the same series: here's a dead guy talking in Call of Duty 2 (and another one), and if you're not planning on sleeping anytime soon, feast your eyes upon the undead human vibrator from Call of Duty 3:
It's like he died while performing a break dance routine and his body simply refused to stop going. Even though he has no legs.
We're not sure who just got served here.
And it happens in other types of games, too: check out this twitching face in Crysis (and another one), this dead guy desperately fanning himself in Far Cry 2 and a dead lady turning her head around in Red Dead Redemption.
Here, we'll leave you with this winking corpse in Infamous: