If the ball was pitched while the batter was in this position then- It was most likely a balk because he pitched while "not facing the batter" according to rule 8.05(f)"(..it is a balk when) The pitcher delivers the ball to the batter while he is not facing the batter".
Rule 6.03 defines the legal batting position of the batter in order to be defined for rule 8.05(f)
6.03 "The batters legal position shall be with both feet in the batters box"
Because the batter was not in a legal position for the pitcher to throw the ball, a balk was called.
This is similar to the illegal "quick pitch" which defines pitching the ball once the batter has taken his legal position in the batters box but before he has been given a reasonable time to get set for the pitch.
If it was not pitched, then of coarse the balk can still be called if time hasn't been called and the pitcher makes a motion while touching/ not touching his plate that would constitute a balk.
I was once in a game where the pitcher would start his pitching motion immediatly once both feet were in the box while the batter was still getting dug in. I argued and won that it was an illegal pitch to pitch that quickly.