The only reason Zimmerman got out of the car was to confront the guy. He told the cops about him. Other wise what happened would be this: Zimmerman got out of the car, did not confront the guy, was going back to the car and for no reason whatever the guy attacks Zimmerman from the back. Don’t think this is what happened.
Zimmerman left the truck in order to maintain visual contact with Martin who was running. The transcript of the call to police proves that. Your last sentence `Don`t think this is what happened` is indicative of the problem with this entire conversation. You are speculating as to what happened. All we have to go on are Zimmerman`s statement, eyewitness accounts, police reports and the transcripts of a phone call to police none of which support your speculation.