In 2008, Major League batters struck out more frequently than they had at any point in the modern era. Until 2009, when a new strikeout rate record was set. Until 2010, when a new strikeout rate record was set. Until 2011, when a new ... well, you get that the strike zone has expanded right along with it.
In his earliest days in office, Commissioner Rob Manfred made it clear he wanted time to determine whether the overall decline in offensive production in recent years is an aberration that will self-correct or if the league needs to intervene. And although run production did improve in 2015, an increasing proliferation of strikes in the lower part of the zone means that, for the first time in two decades, serious consideration is being given to a revised definition of the strike zone.
"I'm not in a position to predict whether it's going to happen or not," Manfred told The Associated Press earlier this week. "I think that the interest in the topic is really driven by the fact that if you look over time, there has been a movement down of the strike zone, largely as a result of the way we evaluate the strike zone with umpires."
Take a look at this heat map of strike-zone data from 2008-15 to see that movement:
Gif: how strikezones have changed 2008-2015
Clearly, the low strike has seen a rise.
Regarding the evaluation process Manfred mentioned, Major League Baseball uses pitch-tracking data to rate umpire performance in a system called Zone Evaluation (or Z.E.). Because these are human beings we're talking about, there is a two-inch buffer built into the system on all sides of the strike zone, and the rise in popularity of the two-seam sinking fastball in recent years means that buffer becomes especially pertinent in the bottom of the zone. A pitch two inches below the kneecap that is called a strike might be technically incorrect, by the letter of the law, but the umpire is still given credit for a correct call in the system.
Given the specifics of Z.E., and the 1996 expansion of the bottom of the zone from the top of the kneecap to the hollow below the knee, it is understandable why an umpire would err on the side of strikes when it comes to low pitches. And with on-base percentage emphasized and hitters seeing more pitches (the rate of 3.82 pitches per plate appearance in 2015 was the third highest since 1990, when STATS LLC began tracking that data), conditions are ripe for more strikes.
And, ergo, more strikeouts.
Looking back, that decision to lower the zone going into the 1996 season was very proactive, because offensive production was just beginning to explode across the Major League landscape. We were at 4.12 runs per game in '92, but that number was up to 5.08 in '99. The question now is whether that production has diminished to the point where it make sense to return the definition of the low zone back to its original wording.
Here's a chart that shows runs per game across baseball in the Wild Card era.

With all due respect to that 0.04-run climb from 2011-12, 2015 provided the first real rise in run production in the big leagues in a decade. For fans who want to see more action, this is encouraging, but not so encouraging that real conversation about the definition of the zone shouldn't be encouraged.
"The umpires have done a great job calling the strike zone as we want it called," Manfred said. "The question is whether we ought to make an adjustment."
I ran the notion of such an adjustment by multiple pitchers and pitching coaches last spring, and it was met with an expected amount of pessimism on their part. But as Dodgers right-hander Brandon McCarthy said at the time, "We're all just an ant colony -- you put something in front of us, we'll all just figure out a way to go around it and something else will emerge in its place."
As recently noted by FanGraphs' Jeff Sullivan, there is data to suggest the umpires are the ones making the adjustment right now, taking note of the increasing popularity of pitch framing and adjusting their technique accordingly. It's merely a theory at this point, though it would help explain, in some measure, the sudden rise in production in 2015.
With a new CBA looming, this is a time in the sport's evolution where any and all worthwhile talking points are being hashed out, both publicly and privately. By year's end, we'll have more data to further inform us the direction offensive production and the strike zone itself are taking, and that will help influence the eventual direction of the discussion.
For now, the rise of the K and the lowering of the zone both inspire the question of what, exactly, a strike should be.