Mr. Zimmerman called the non emergency number to report a suspicious person. When set upon by said suspicious person and pinned to the ground, retreat was impossible so he stood his ground. Not that he had a duty to retreat. So much went wrong for the whole mess regardless of the results. No gun, Mr. Zimmerman would likely be dead or brain damaged. The perp would probably have walked.
He did not stand his ground, he was grounded and defended himself. No stand your ground law was cited by defense.