It's the inner caveman realising he's only just got away from that sabre tooth tiger and maybe he won't get so lucky next time.
I know that sounds pretty frivolous but the need for humans to run all goes back to pretty stressful situations such as hunting, which is dangerous and puts you at a high risk of injury or getting away from something hunting you. So maybe it's quite natural.
Try standing up as straight as you can. Now are your knuckles still on the floor. :-)