It all comes down to psychology. I believe Zimmerman got out of that truck because he was feeling brave and wanted to try and see where Martin ran to. Martin saw him from the dark and waited for him to get off the phone. He then got in his face and sized him up asking "why are you following me". At this point, Zimmerman still believed this guy was a burglar and rather than saying "I am neighborhood watch, just wondering what you are doing around here" he chose to play dumb hoping to stall him until the cops showed up. After that, it is impossible to know what exactly happened. Both scenarios are somewhat illogical: Why would Martin start pummeling him unprovoked, and why would Zimmerman start a fight with a taller/fitter "Burglar". The answer to that 2 or 3 seconds which began the physical confrontation will never be known for sure.