Martin initiated the contact, he didn't like being followed and he let Zimmerman know it
and does anyone have proof Zimmerman continued "following" (he wasn't following him at that point, they were walking towards each other) Martin after the dispatcher told him to break it off? He said he turned around to head back the way he came from his truck and Martin came up behind him.
So according to your understanding of the law Martin was standing his ground when Zimmerman asked what he was doing there, but Zimmerman was not "standing his ground" when Martin knocked him down and bashed his head on the concrete?