Martin was being pursued by a person in a car. This person was following him. We know this as fact due to the police call.

Martin bolted, Zimmerman followed. We know this as fact due to the police call, and the car noise when Zimmerman left the car.

We are then taking Zimmerman word for what followed after words about losing Martin.

That said, if a person is following you by vehicle, followed by foot complete with running, during rain, this is a fight or flight situation, and when a strange man is chasing you it is likely that Martin would have "defended" himself and started a fight for a strange man is chasing him.