When 1B Umpire Dan Bellino called Reds batter-runner Nick Senzel out, he invoked Official Baseball Rule 5.09(b)(9), which states that "
any runner is out when they pass a preceding runner before such runner is out."
This infrequent play generally occurs on fly balls to the outfield when less than two are out as the existing baserunners try to anticipate whether the fly will be caught—in which case they may be forced to advance—or whether the ball will land safely—in which case they may appreciate a head start while running the bases.
In other situations, a runner beginning play on first base (R1) might be attempting to steal as part of a hit-and-run play, and suddenly find themselves forced to retreat as the batted ball to the outfield may or may not be caught by an outfielder.
For this play, Cincinnati batter Senzel's fly ball was
not caught by Baltimore, but baserunner R1 Kyle Farmer, who had advanced to second base, retreats back toward first base, mistakenly believing the ball had been caught, as Senzel stands between first and second base.
The runners pass each-other—by rule, no matter which runner is primarily responsible for the passing, the trailing runner is always out when passing occurs and thus said to be the "passing" violator regardless of which offensive player erred—and Bellino properly enforced OBR 5.09(b)(9) by keeping the ball in play and signaling the back runner, BR Senzel, out for passing the preceding runner, R1 Farmer.
Note that the defense does not need to tag the violative runner (Senzel) for an out to be called. The out is immediate and automatic upon the passing occurring, and Bellino properly called the out before the Orioles tagged Senzel. This would be important for time play purposes if there had been two out and another runner attempting to score.
Video as follows:
