Given that Zimmerman had 100 pounds on the teen, and a gun, he probably figured it was going to be quick work and he'd be a hero (cop wannabe is already a label that's all over the news). Again, speculation, but I see it as possible that Zimmerman grabs the kid's hoodie and pulls him back to keep him from escaping. This might have been accompanied by some choice words, if Zimmerman's attitude on the 9-1-1 call is any indicator. Now, MAYBE he turned around with a "you stay right there; I'm on the phone with the cops" and he turned back towards his vehicle. That seems rather unlikely given that he thought the teen was such a danger. It sounds much more like an attempt to separate the actions, which would lend more credence to a self-defense claim. What I find more likely is that a kid who was already being followed, and possibly got called a name, decided that some fat jerk was not going to get away with that, and socked him in the nose. Zimmerman's nose was broken. With that kind of injury it's very likely they did go down and start fighting. What they haven't really released, or at least the news has not mentioned, is how far apart they were when the gun actually went off. Were they struggling on the ground? Did the teen get up and run away? Did he get up and gloatingly stand there?
The "violent assault" included a bloody nose, a little blood on the back of the head, and a cut. All of those injuries would be explained by the purely speculative account above, yet it would be easy to wedge the eye witness account in there. He did not appear to see the shooting. He saw the two fighting, then in a little bit he saw a corpse.
If the cops showed up and decided not to arrest the person who followed, fought with, then shot your child, you'd probably be a little mad, too.