George Zimmerman -- Shooting Was Unprovoked ... He Chased ME Down (UPDATE: 911 Call)

George Zimmerman was fleeing for his life when a man opened fire on his truck ... this according to Zimmerman's attorney who's calling BS on the shooter's claim it was self-defense.

Matthew Apperson admits firing one shot at Zimmerman's truck Monday afternoon in Lake Mary, FL -- and Zimmerman's attorney Don West tells TMZ ... George did nothing to set off the attack.

West says Zimmerman was driving down the highway when he noticed a car behind him flashing its headlights ... and when the driver stuck his head out the window to yell something -- Zimmerman recognized it was Apperson.

As we've reported ... Apperson and Zimmerman have a road rage history.

West tells us Zimmerman made the first available u-turn to get away from Apperson -- who also turned to follow him, and then pulled up on the right side of Zimmerman's truck ... and opened fire through the passenger side window.

As for Apperson's claim of this being a self-defense shooting -- West says his client NEVER brandished his weapon ... and cops confirm he never fired it either.

Zimmerman was treated for minor injuries at a hospital and released. Apperson has not been arrested.

The 911 call from the scene is interesting though. It was made by a guy Apperson flagged down, and the caller says ... "He had to shoot at somebody."

Sounds like Apperson was already telling people it was self-defense.