you see that Zimmerman is upset when Trayvon begins to run away after seeing Zimmerman pursuing him. He then gets out of the car it sounds like to chase him down, and the officers tell him not to.
Zimmerman was the aggressor.
This is consistent with Martin's girlfriend who was on the phone at the time, who said Martin was worried about this guy following him, and was trying to get home quickly.
Then she hears the confrontation, and Martin screaming "get off, get off."
Put that together and it seems Zimemrman tried to apprehend Martin for the police, because as he told the police in the recording, "these assholes always get away."
Here you see her witness testimony, is that Martin is running away and Zimmerman is pursuing him (consistent with the 911 call). Martin asks why he is following him, there is a scuffle, Martin says "get off me" and then the phone call ends.