As you know, baseball’s experiencing a home-run surge. It hasn’t affected all pitches equally. Home-run rates against high pitches are up, but they’re lower than they were in 2008-2009. So, there’s still a feeling of some normalcy. Home-run rates against low and middle pitches, though, have skyrocketed, relatively speaking. Middle pitches go for homers most often, and that’s what you’d expect, but also, compare the other two lines. Since the start of last season, low pitches have gone out more often than high pitches, on a per-swing basis. That’s not how things used to be.
The easy, over-simplified explanation: Pitchers have increasingly been working down. Hitters have worked to combat that by trying to elevate pitches down. Anything you’ve ever read about a hitter trying to hit more fly balls has involved that hitter trying to elevate pitches around the knees. Pitchers worked low, so hitters started to work low. Hitters got better lower in the zone. This doesn’t capture everything, but I do think it makes overall sense.