Here's my problem. Can someone explain why Martin would attack Z if Z was doing nothing wrong?

[Bjorn Against]

69. Who was on top is disputed, but no one saw them go to the ground

My thought on what happened, and I acknowledge this is speculation but it fits with the evidence, Zimmerman initially grabbed Martin and Trayvon struggled to escape, during this struggle they lost balance and fell to the ground, during this fall Trayvon came down on top of Zimmerman landing on top of his head and knocking it back into the ground which would explain Zimmerman's wounds. This is the story that makes the most sense to me, but I acknowledge it may be wrong. I believe very strongly that Zimmerman was the aggressor though, his phone call to police proves that to me and I could believe a different story which showed Zimmerman as the aggressor, but I could not believe a story that shows Martin as the aggressor because all the evidence shows Zimmerman started the confrontation.