As we know from the timeline (at least what those of us who actually follow the real facts of the case know), Trayon Martin and Zimmerman looked at each other, Martin ran away, and Zimmerman got out of his car. A few minutes later, Martin and Zimmerman were in a fight. In between those two events, Martin spoke on the phone to a girl identified only as “DeeDee” who happened to call him.
If what really happened is that Martin went back to confront Zimmerman after he evaded him (which is likely), then there’s a good possibility that the phone conversation with DeeDee is what caused Martin to change his mind.
I think there’s a possibility that DeeDee is lying about the whole incident, and that what really happened is that she told Martin to go kick Zimmerman’s ass after Martin told her that some guy who looks like he lived there was looking at him like he didn’t belong there.
This version of events would explain why DeeDee never told anyone about the phone call until the Martin family or Ben Crump found the call on Trayvon’s cell phone statement. It’s because she was complicit in getting Martin killed. She told him to beat up someone who turned out to be packing heat. You can see why she'd be motivated to make up a story that casts herself, as well as Martin, in a much more positive light.
There’s a long history of black women who lie about some incident in order to incriminate white people. I am thinking about Nafissatou Diallo who lied about the circumstance of her encounter with Dominique Strauss-Kahn, Crystal Mangum who lied about being raped by Duke lacrosse players, and Tawana Brawley who at the age of 15 lied about being raped by a bunch of white men.