I believe that is because of the forward progress rule. When a player catches the ball while coming back to the QB and is hit immediately driven backwards by the DB, then the ball is spotted at the point of the catch not where he's eventually tackled even out of bounds. Since the spot is forward progress, the clock keeps running since that is in bounds.
I'm not sure about if the player makes the catch and runs out of his own power backwards, but likely he'd turn and run for the sidelines parallel or forward if the DB isn't right on him.