Quote from AAAintheBeltway:
The short answer is that Zimmerman was apparently returning to his SUV when the kid jumped him.
Obviously he was following him because he was a black teen who he didn't recognize, wandering around a restricted area. Some would call that racism, others would say it reflects reality. From the fact that Zimmerman was a frequent caller to 911 for a number of fairly minor issues, it is speculation tosay he wouldn't have done the same thing if it had been an unfamiliar white teen.