https://phys.org/news/2018-07-home-surge-baseball-statistics-hot.html

And then there was a massive investigation requested by the MLB commissioner, who asked 10 scientists to find out what was going on. They tested a lot of balls and concluded it was a case of reduced drag combined with the launch angle of the ball coming off the bat.

But Jason Wilson, a statistician at Biola University in Southern California, has a different explanation. The poorer the pitch, the easier it is to whack a home run—and the quality of pitching between 2015 and 2017 had gotten worse if you broke a pitch down into measurable components and then measured pitching quality over time. Wilson called this measure "Quality of Pitch" (QOP).