A theory: Martin's hands could have been thought of as a deadly weapon. Skilled martial artists and CQC experts need to do so, and Zimmerman could have made that (false) assumption.
It's worse than that. A "reasonable fear of death or bodily injury" can be claimed as soon as Martin touched him. Any fight contains a "reasonable fear of death or bodily injury". Therefore, anyone can argue it in any violent confrontation, because yes, there is a "reasonable fear of death or bodily injury". Remember, bodily injury is a vague term. Is bodily injury pissing blood? Coma? Broken bones? Bruised bones? A scrape? While chances are that last one wouldn't work, the rest are all bodily injuries.
Hands, regardless of whose they are, are never considered deadly weapons. And even if they were, Zimmerman would have to have reason to believe that Martin was a skilled martial artist. Since Zimmerman had no idea who Martin was, there is no way he could have had that belief.
PostHuman, the phrase is "serious bodily injury." Legally, that phrase means pretty much "organ failure" or "injury likely to lead to death."