I will suggest a scenario. It may be true but of course there is no way to tell right now. I will say I have seen this exact same thing happen.
Zimmerman sees Martin acting suspiciously. They eye each other. Zimmerman makes some attempt to investigate then eventually loses sight of him and relaxes.
Eventually Zimmerman goes about his business. Suddenly from absolutely nowhere he is attacked from behind, knocked down and starts to take a vicious beating.
That is as far as the scenario goes.
That might be what happened. Hopefully a truthful version will be put to the Grand Jury, and they will believe it, and act accordingly.
But in this racially-charged atmosphere, I think Holder will put his thumb on the scale, hard.