First off it wasn’t his father’s house. Second, if someone is following you and you are uncomfortable about it, use the cell phone to call the cops and then go home quickly. Its risky to swing around and attack the person from behind.
It was his father’s girlfriend’s home - but the father was staying there, too. As for being attacked from behind, that might be Zimmerman’s story, but there are no witnesses who saw it start.
According to reports (not confirmed or denied by the Sanford Police Dept.):
"Zimmerman told police that he lost sight of Martin and was walking back to his SUV when Martin approached him from the left rear, and they exchanged words.Martin asked Zimmerman if he had a problem. Zimmerman said no and reached for his cell phone, he told police. Martin responded, "Well, you do now" or something similar and punched Zimmerman in the nose."
Unless his official statement contradicts what has been reported (which is possible), Zimmerman didn't claim that Martin attacked him from behind.