If you watch the sequence of pitches to Mateo, up until the 3-1 pitch he used the hybrid windup. He came set from the stretch after receiving the sign and then proceeding to throw the pitch from the windup (moving his left foot first to to deliver the pitch instead of lifting his leg to deliver the pitch). By doing this, this establishes his move. The reason the balk was called on that 3-1 pitch is that he didn't use the hybrid delivery, he came set and then lifted his left leg and delivered the pitch which was different from the hybrid he had used on the previous pitches to Mateo. Once he lifted the leg, the balk was called right away because his delivery was different. Had he been in the traditional windup up until that point then it would have been nothing but because he used the hybrid delivery prior to that pitch, that's why the balk was called.