Rule 8.01 Comment: Pitchers may disengage the rubber after taking their signs but may not step quickly onto the rubber and pitch. This may be judged a quick pitch by the umpire. When the pitcher disengages the rubber, he must drop his hands to his sides.
Pitchers will not be allowed to disengage the rubber after taking each sign.
Rule 8.05(e) Comment: A quick pitch is an illegal pitch. Umpires will judge a quick pitch as one delivered before the batter is reasonably set in the batter’s box. With runners on base the penalty is a balk; with no runners on base, it is a ball.
The fact that the umpires did nothing indicate they did not feel the batter was at a disadvantage and was reasonably set. Still its a judgement call by the umpire.
The fact that he disengaged the rubber and did not drop his hands to his side is a balk in my book. But I am interested in the pro's take on it as well.