First of all, the "chase" term you use is one you're making up, not backed up by anything in the call transcripts. Zimmerman doesn't look to be in good enough physical shape to be outrunning a teenage athlete either. Martin looks like he's in good enough condition to run rings around Zimmerman, so thinking he was afraid of that guy "chasing" him seems fanciful.
As far a valid reason? Sure. When that suspicious stranger started running away, it made him look even more suspicious. Zimmerman most likely wanted the police to actually have a chance of finding Martin so they could talk to him. Zimmerman wanted to keep him in sight so he would know where to point the police to when they arrived.
I don't like Mitt but I could never vote for Rick. Ever.
Brokered Convention Please. I have a gut feeling GOP will out raise Obama bigtime.