I've hard a particular 'scenario' in my head of what probably happened, but it's baseless so I haven't shared it. I'll share in now, just for conversation - but I don't have anything to support this so I don't put any real weight on it.
Zimmerman catches up to Martin, and words are exchanged. Martin decides to get a bit aggressive, tells Zimmerman to shove it - maybe shoves him too. Zimmerman maybe shoves him back, regardless - something as benign as a shoving match occurs. In this however, a shove occurs hard enough to knock Zimmerman to the ground and smack his head on the sidewalk (maybe it was a punch to the nose, maybe it was a shove to the nose, maybe it was neither and his nose actually wasn't bleeding). Regardless, Zimmerman gets up (maybe knocks Martin to the ground) and pulls out his gun. He is furious that he got wounded and is in some sort of adrenaline fuelled rage. This is where the screaming begins. Maybe he gets ontop of Martin and starts cussing him out with the gun in his face or chest or something, threatening him with the bravado that happens in these situations (Do you know who I am? Etc) - maybe his intention was never to shoot, but just to scare him shitless. Obviously Martin is fucking frantic and tries grabbing for the gun at this point, or with some adrenaline rush, is now substantially stronger and is fighting back in some way (all the while screaming for help) - eventually, Zimmerman gets scared because this kid is a bit too strong with the adrenaline pumping through his veins - so he takes the shot.
Again, I'm not even 50% sure on this scenario, this is just what my original idea of what happened was.