When I picture this, if Trayvon were afraid of GZ - that is, believed GZ intended to knife him, kidnap him, rob him, etc., and knowing that Trayvon wasn't armed, I don't think he would have verbally engaged GZ, and asked him what his problem was. If he truly feared for his safety from GZ, I think he would have broken into a run and gotten away and not forced an encounter.
I think he knew what GZ was doing, and it pissed him off so he took the first swing. i believe Trayvon knew GZ suspected him, and was following him to watch him, and so Trayvon attacked him first verbally and then physically.
That's my humble opinion only. I can't see verbally engaging someone who you are truly afraid of, if you are unarmed, if you have the chance to get away which Trayvon certainly did.