The snake has heat-sensitive pits at either side of its face, which it uses to detect threats and let's face it, if you're close enough for your body heat to be detected, you're close enough to be considered a threat. Oh, and also what the hell are you doing standing so close to a venomous snake's face?
These heat-sensitive pits are capable of detecting a threatening presence for hours after death, which means the snake may continue to defend itself, zombie-style. And yes, this applies even if the body is no longer attached. So anyone dumb enough to poke and prod it to assess its level of deadness may quickly find themselves with a sudden increase in the level of pissiness of their pants when the snake's movement sensors kick into action.
But don't worry, it's not all doom and gloom, because a snake's venom loses its toxicity after its death. Except that's a total lie, which means that getting bitten by a dead snake can make you just as dead as getting bitten by a living one, but add to the excruciating pain the severe humiliation, because who the hell loses a fight to a dead animal?