[palmer]

when he was caught from behind, knocked down from a single punch by Trayvon

No, the story says approached from behind, then they exchanged words, then Martin punched Zimmerman in the nose (all this is according to Zimmerman, but it fits the facts).

[caww]

then they exchanged words,

What words?

I read that Martin said to Zimmerman..."You have a problem"...and Zimmerman said ..."No"...and then Martin said ,"You do now" and took Zimmerman down with a blow and continued to bash his head on the sidewalk.