Maybe he was nailing up into the underside of a structure while looking down at the work, and the gun kicked off the work and fired a second nail (possible if he had disabled the tip safety and was running the nail gun in "trigger only" mode, and it kicked in his hand in such a way that the trigger was actuated twice) - at which point it wouldn't have taken much of a "recoil angle" to get one into his mouth.
(The news report merely said that "the gun backfired and he didn't realize it had fired a second nail...")
-dave w