Is there any testimony by Zimmerman that gives his reasons for being suspicious of Martin? The call to police doesn't seem to provide any compelling reasons.
If, and I stress if the girlfriend's statement is accurate: Zimmerman, is driving and sees a kid walking slowly in the rain. He watches him for a second to see why he is wandering around in the rain. Martin says to his girlfriend, there's this guy watching me and he pulls his hood over his head. Given the neighborhood has had a series of recent robberies committed by young black males (according to Zimmerman's neighbor), Zimmerman may have seen a kid notice he is being watched and pulls his hoodie up. Zimmerman may have thought he was hiding his identity. Just a possibility based on what has been added to the story since the beginning.