Straight from the rulebook,
"If a batter strikes at a ball and misses and swings so hard he carries the bat all the way around and ,in the umpire's judgment, unintentionally hits the catcher or ball in the back of him on the backswing, it shall be called a strike only (not interference). The ball will be dead, however, and no runner shall advance on the play"
It's very cut and dry. The umps made the right call.