Let's talk about why Trayvon would slam Zimmerman's head when he's already in a dominant position. My guess would be, his hands got tied up or rage.
Hands Tied Up:
So if Trayvon was measuring his punches, he might have one hand on Zimmerman's collar or neck area, with one of Zimmerman's hand trying to control it. Now maybe Zimmerman is able to grab onto Trayvon's striking hand, and tuck it into his chest and neck area, preventing Trayvon from striking. To shake his hands free, I could see Trayvon lift up, and come down to slam Zimmerman's head.
Rage:
Someone is in a rage and just wants to inflict damage at any cost. Like if the other person just goes full fetal mode, they lock up. So they can be picked up from the arms and dropped onto the ground. I guess Trayvon could "snap" somehow, but I can't think of any reason that doesn't involve provocation from Zimmerman touching him or saying something.
Anyhow, I'm trying to imagine into this "fight" when Trayvon would scream for help. My guess is while Trayvon is trying to shake free, slamming Zimmerman's head, Zimmerman can feel his grip slipping, and decides to go for the gun. So with his gun drawn, it's Zimmerman's right (usually dominant hand) hand vs Trayvon's left (usually less dominant) hand. It's at that point Trayvon could be screaming for help. But that would mean a pretty long struggle with the gun drawn. It'd make more sense if Zimmerman just drew and fired right away while Trayvon was slamming his head. But...... aw fukk it. Forgot to consider the DNA that should have transferred with so much hand on hand contact and etc. But shyt, I ain't gonna delete this post now, I've typed too much. Just gonna come back to it later.