It might be. It depends on the holster. One of the main purposes of a holster is to shield the trigger from being pressed while still in the holster. But if the gun fired while in the holster, then Zimmerman would have been shot in the leg, rather than Martin getting shot in the chest.
We don’t know where Martin was except on top. If they were fighting over the gun as Z said, and the gun was at his side in the holster when the first policeman arrived, I question if the gun got totally out of the holster.
On FOX just now they played one of the 911 calls.(from a woman) Voice could be heard calling for help and during it the gun went off.