Electric charges repel, and they exert force on objects that contain those charges. If you have a large amount of static electric charge on you, it will spread out over your skin so that any given charged particle on your body will be as far away from any other such particle as possible. Thus, they will flow down to the ends of your hairs. However, your hairs are close to other hairs, so causing these hairs to stand up is the farthest distance from other hairs as they can get.