It's an old trick that a few of my opponents have tried to play on me (you might even call it an underhanded swindle). I once had an opponent who hung his queen with 25 minutes to go. He offered a draw and when I declined, he called me a few names and said I would have to wait until the clock ran out. Sure enough, with five minutes left he made his move.
It's not stalling, they're just trying to take advantage of their opponent's inattention.
Yeah I'm not saying what your opponent did was cool, I'm just saying he did beat you fair and square because you were careless to another level.