A couple of recent articles have looked into the increased rate of strikeouts per game.
In the New York Times, Tyler Kepner has an article titled "Swing and a Mystery: Strikeout Rates Are Soaring". This one has a sidebar article "Strikeouts on the rise", which includes an interactive chart displaying the changes over time (including an optional overlay for a selected team).
Some explanations offered--none conclusively--include increased use of relief pitchers, batters are more likely to swing aggressively with two strikes, better information available to pitchers, and pitchers throwing more strikes (walk rates are the lowest they have been in 20 years).
At Beyond The Boxscore (part of SB Nation), James Gentile takes a look at the rise of the called strike. Gentile notes that pitches per plate appearance (PA) have risen, and it's been called strikes and foul strikes per PA, not swinging strikes, that have risen. Gentile suggests (in common with some of Kepner's ideas) that batters are being less aggressive, and more patient.
So far, the research is only scratching the surface. I'm sure we will see more in the future.
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