MLB players suddenly started hitting more home runs in 2015 and they haven't slowed down since
Through about one-fifth of the Major League Baseball season, the players are on pace to hit 5,730 home runs, which would break the record of 5,693 home runs hit during the 2000 season in the heart of the so-called "steroid era."
What is interesting about this surge is that something seemingly changed, either prior to, or during the 2015 season. That's when the fly balls suddenly started flying out of ballparks more often.
During the first half of the 2015 season, 10.7% of all fly balls went over the fence for a home run. That was up from the 2014 season, but it was still in line with what had been seen in MLB over the previous five seasons. But then the rate shot up again after the All-Star break, reaching 12.1% of all fly balls. Was that an anomaly? Nope. The rate rose again early in 2016 and has remained steady ever since.
It is unclear what has caused fly balls to suddenly start traveling farther, however, Nate Silver's FiveThirtyEight pointed to some circumstantial evidence that the baseballs are "juiced," that something changed in the construction of the baseballs used at the MLB level.
One of the more compelling pieces of evidence is that home run rates in triple-A have historically gone up and down in a similar fashion to MLB. However, while triple-A home runs were going down from 2012 through 2014, they continued to go down in 2015 and 2016 at the same time home runs in MLB were on the rise.