https://www.sciencedaily.com/releases/2018/07/180723142926.htm

What was going on? For a stats-mad sport, the mystery was irresistible. There was the theory of the "Juiced Ball." Some subtle, possibly unintentional change in the manufacturing process had given balls just enough extra bounce to change history. Then there was the batter approach theory, which speculated that just a little bit more of an uppercut swing -- perhaps in part due to defensive shifts -- was giving the ball extra lift. Maybe batters were just cranking it as hard as they could and going for home runs given this shift to stronger defensive tactics?

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.