How do we know Trayvon threw the first punch?
We don't know it, but it's the only story we have to go off of and unless Zimmerman started a fight only to be quickly laid out by a skinny kid, then I'd say it's likely.
Considering Zimmerman's comments on the 911 call before the altercation, it certainly is possible. I wouldn't just assume Martin attacked Zimmerman, maybe it started with an argument, Martin tried to get away, then Zimmerman grabbed or pushed him, prompting Trayvon to fight back, just as an alternative scenario. We don't really know what happened before the fight started at all except what Zimmerman said, but remember his story changed several times.
Just a consideration, if Zimmerman did start the fight, and was overpowered by a younger more athletic Martin, would he still be justified in shooting him?
Also just to clarify, I agree with the verdict according to how the law is written. The burden is on the prosecution in a criminal case to prove that Zimmerman was not in danger of his life in order to beat stand your ground, and unfortunately they couldn't. However, in a civil case the burden would be flipped, Zimmerman would have to prove his life was in danger, which could have a very different outcome.