Three weeks into the major league season, scoring dropped half a run per game through the same point in the previous season. What caused this drop? It could be the umpires, but looking at heat maps of called balls and strikes, there is no discernable difference in the frequency of strikes called outside the strike zone and balls called inside the strike zone. The same errors exist in the same locations.
Batter selectivity then came into question. Again, looking at heat maps, the only bit that looked different was that maybe batters were taking more strikes down the middle of the plate, and maybe they were taking more high strikes. These were minor differences at best, probably accountable due to the sample size early in the season.
Combing through the data, the one thing that stood out was a change in the frequency of certain pitches:
Fastballs are down, and pitchers are replacing them with cutters and sinkers. Batters are chasing those pitches out of the strike zone.
This season, when batters swing at pitches outside the zone they produce a .191 wOBA. When they swing at pitches inside the strike zone, their wOBA jumps to .323. Pitchers as a whole changed the way they approached batters, and that caused them to swing more at pitches with a low probability of success.
The changes hold up across the two leagues as well, although the AL is substituting more sinkers and the NL more cutters.
Batters are seeing a different mix of pitches than usual, and they'll need to adjust if offense is to recover.