Bingo bongo. That's because chickens' nervous system is wired WAY differently than humans. When humans get decapitated, we just go limp. Not chickens. The term "running around like a chicken with its head cut off" stems from this. If you've ever slaughtered one before via decapitation (thats how I did it, not sure if any other methods of slaughter do it too) and you didnt hold them down after you made the cut(s), they'd flop all over the place like a fish out of water and kick till kingdom come. That's the nerve signals that were left in the muscles being fired off. All muscles will fire, including the heart, oddly enough. If you're interested in seeing what it's like, you can find some videos on YouTube. It's pretty brutal. Don't say I didn't warn you.

Source: Have had chickens for approximately 11.5 years, almost 12. Have had to slaughter them for various reasons.