Nick Pivetta Called for Balk Despite Declaring Hybrid

You asked us to look at Red Sox pitcher Nick Pivetta's balk that allowed Blue Jays baserunner R3 Kevin Kiermaier to score. With Pivetta clearly declaring hybrid (full windup) prior to Bo Bichette's at-bat, why did umpires call Pivetta for a balk on his third pitch?

With Pivetta, Boston manager Alex Cora, and others still confused on the call after the game, we move beyond the simplest explanation of umpires calling a balk for taking a step back (as in windup) without declaring hybrid (as opposed to Set Position), because it simply wouldn't make any sense for the umpire crew to suddenly disregard Pivetta's hybrid declaration on the third pitch of the Bichette at-bat.

Instead, it appears Pivetta was called for buckling his free knee ("knee pop") prior to lifting his foot to step back in windup position. We usually see knee pop balks occur prior to jump turns on pickoff attempts, not on actual pitches, but the rules do allow balks to be called whenever a runner is on base, whether or not a pickoff is actually attempted.

In conclusion, this appears to be a balk call due to a knee pop which, your mileage may vary, was quite minor.

Video as follows:

Alternate Link: Pivetta balked on third pitch of at-bat, but was it a hybrid issue? (CCS)