The Play: With two out and a runner on third base, Cubs batter Michael Busch stepped in for a 2-1 pitch from Diamondbacks pitcher Eduardo Rodriguez as Chicago baserunner R3 Pete Crow-Armstrong danced down the third base line. Rodriguez was pitching from Windup Position, having declared Windup while standing in a Hybrid stance (footwork indicating Set Position, but opting to pitch from the Windup), giving R3 PCA a little more time for baserunning antics near 3B, which pitcher Rodriguez noticed mid-delivery and rushed his motion home, turning the 2-1 pitch into a pitchout of sorts to catcher James McCann, who exited his box to receive the ball.
Chicago campaigned for a balk, while 3B Umpire Malachi Moore gestured for the crew to convene. After consultation with 1B Umpire Mike Estabrook and 2B Umpire Lance Barrett, HP Umpire and Crew Chief Alfonso Márquez explained via public address microphone that the crew deemed there was no balk, drawing Cubs manager Craig Counsell for an argument.
Was This is a Balk? To determine whether this was a balk, we first narrow down the types of balks this could conceivably qualify as. Pursuant to Official Baseball Rule 6.02(a)(1), "if there is a runner, or runners, it is a balk when the pitcher, while touching their plate, makes any motion naturally associated with their pitch and fails to make such delivery"; however, pitcher Rodriguez did fire home.
We next investigate the so-called catcher's balk, aka OBR 5.02(a), which states, "the catcher must stand with both feet within the lines of the catcher's box until the ball leaves the pitcher's hand. PENALTY: Balk."
Upon closer inspection, this is a balk pursuant to OBR 6.02(a)(5): "the pitcher makes an illegal pitch" because of Rule 5.07(a) Comment, which states, "The pitcher may not take a second step toward home plate with either foot or otherwise reset their pivot foot in their delivery of the pitch. If there is a runner, or runners, on base it is a balk under Rule 6.02(a); if the bases are unoccupied it should be treated as an illegal pitch under Rule 6.02(b)."
Replays indicate Rodriguez in his rushed delivery reset his pivot foot by stepping off the back of the pitcher's plate/rubber entirely, this disengagement (inadvertent or otherwise) making this an illegal pitch. This was a balk that was not called.
Video as follows:









