Two Potential Reasons for Lower Scoring
This year, scoring is down by almost a quarter of a run per game.
At the beginning of the season, Mike Fast showed that fastball velocities were rising. FanGraphs data indicates a continued upward trend. I spotted only two pitchers from 2009 who threw 96 MPH and were out of the league in 2010 (Juan Morillo and Tyler Yates), while there were about a dozen rookies who came in throwing that (Aroldis Chapman, Jordan Walden, Stephen Strasburg, Dan Cortes, Andrew Cashner, Alexi Ogando, Joe Bisenius, Jhan Martinez, Chris Sale, Greg Holland, Sergio Santos, Gregory Infante). I suppose it's normal for there to be more hard-throwing rookies entering the league than hard-throwing veterans retiring. Still, only 30 pitchers averaged 96, and that nearly half of them were rookies sounds exceptional.
Also, I checked to see whether the strike zone has changed. Red zones indicate a higher rate of called strikes, and blue lower.
I'm not too confident in drawing any conclusions from this, but it appears that umpires might have gotten better at calling strikes on pitches at the knees.