Bullets coming apart from jacket failure. Could be from velocity and spin speed (highly unlikely at .308 speeds with standard .30-caliber projectiles), could be a bad batch of bullets (more likely) or some bullets got damaged while being seated into the cases (also more likely).
I've never actually seen a puff of smoke when one goes, but I've shot at enough rifle matches I've witnessed the magical disappearing bullet more than once. The shooter will call it down the middle and we'll look for a mark, only to have a miss come back.
NRA Master - Highpower Rifle, Across the Course
NRA Expert - Highpower Rifle, Mid-Range Prone