I tend to think Martin's behavior when approaching Zimmerman's truck was the catalyst for this whole event. GZ undoubtedly perceived him as suspicious from the start and I think Martin realized pretty quickly he was being "watched". However, Martin's response is what elevated GZ's suspicions to another level.
Based on what was described in the police call, Martin's approaching seemed unsettling, perhaps threatening, maybe confrontational. Although nothing was verbalized, his mannerisms seem to speak of either trying to spook GZ, or perhaps provoke him.
Obviously this startled Zimmerman, he wanted the cops there ASAP. Through the eyes of someone watching over the neighborhood he saw someone cutting into the development through people's yard, instead of the gate. Possibly looking around at houses aimlessly in the rain and then staring at Zimmerman, approaching him and doing what could have very well been a gesture that he had a weapon in his waistband AND then inexplicably running away after.
Lots of people make the obvious assertion that none of this would have happened had Zimmerman just stayed in the car, but then ignore Martin's actions. However, if Martin just continued to walk home casually, no circling, no staring, no gestures I don't think Zimmerman would have left his vehicle or even have found merit in following up with police after.