My guess, certainly not evidence, is that Jeantel urged him to kick the crazy ass cracker's butt.
OR... he had hidden his ‘break in’ tool in the bushes right after he first saw Zimmerman, and after he got home safe, he decided to go back and get it. That’s why he came popping out of the bushes and confronted Zimmerman. He was in the bushes trying to find it when he heard Zimmerman on the phone talking.
Well, he was probably mad by then, being ‘interrupted’ again, so he decided to DECK THIS FOOL.