Why did Dellin Betances decline during the second half of 2015?

In 2014 and 2015, Dellin Betances put together one of the best two year stretches of relief pitching in history. Over his first two full seasons with the Yankees, he maintained a 1.45 ERA and struck out 39.5% of the batters he faced. Betances managed these feats despite easily leading all relievers in innings these past two years, totaling a startling 174 innings. To dominate so thoroughly, so frequently, during a time when the best relievers are often reserved exclusively for three-out save situations, is practically unheard of. In fact, during the wild-card era, no reliever has posted a season with more innings and a better ERA+ than Betances has had in both 2014 and 2015, according to Baseball Reference:

Yet, during the second half of 2015, Betances exhibited notable signs of decline. Betances actually saw a slight fall in his ERA from the first half to the second half (1.53 to 1.46), but there were hints of trouble elsewhere. He limited opposing players to a minuscule .125/.227/.189 line before the All-Star break, a line that swelled to .194/.313/.313 after. That line is still strong, but batters were hitting for noticeably more power and drawing more walks against Betances. Also, while his ERA remained low, his ERA estimators worsened in the second half, as his FIP (from 1.75 to 3.40) and xFIP (2.01 to 2.92) both were much higher during the second half.

We can turn to his batted-ball profile for answers. According to FanGraphs, Betances yielded soft contact on 37.4% of batted balls in the first half, a number which fell to 20.0% post All-Star break. This led to a second-half spike in both his medium contact rate (from 42.2% to 50.7%) and hard contact rate (20.5% to 29.3%). So, while he wasn't giving up more runs, Betances seemed to be pitching worse late in the season. What changed?

Betances has dominated throughout his career using just two pitches: an upper-nineties fastball, and a knee-buckling curve. Last year, as the season wore on, Betances saw hitters start to square up on those offerings; in this case, the heater. According to Brooks Baseball Pitchf/x data, his fastball velocity wasn't a problem. Betances' fastball actually gained velocity later in the season, peaking at an average of 98.21 MPH in August, compared to a relatively paltry 95.95 MPH average in April. However, this didn't stop hitters from getting better results against his fastball later in the season. Most notably, Betances grew increasingly unable to achieve swings and misses with his high heat:

Batters were having much less trouble in the second half getting the bat to Betances' fastball, with his whiff rate on four-seamers dropping from nearly 20% in May all the way down to 10.81% in September. However, a drop in whiff rate alone would not be enough to torpedo the effectiveness of his fastball; batters would have to prove that they could do damage with that extra contact. Unfortunately, they did:

Here, we see the slugging percentage opposing batters managed against Betances' fastball. For much of the season, hitters were having no luck hitting the pitch for power, but in August, they slugged .684 when Betances threw a fastball. That mark spiked all the way to a shocking .818 in September. Moreover, opposing hitters posted a .110 batting average against his fastball in the first half, compared to a .328 mark in the second half. The sample sizes on these figures are admittedly small, as Betances only pitches anywhere from ten to twenty innings in a month. However, the trend is still clear; in the second half, hitters were not having as many problems putting Betances' fastball in play, and when they did, they found better results.

Strangely enough, this decline only occurred for Betances' heater. His curveball was dominant no matter the stage of the season. Hitters posted a .128 batting average against the curveball in the first half, with that figure actually falling to .078 in the second half. For the season as a whole, opposing batters posted a .103/.172/.154 line against Betances' curve. Whatever struggles Betances was having, they were confined to his fastball.

Why would Betances' fastball lose so much effectiveness, while his curveball remained excellent? He may have worn down somewhat, under the weight of the most strenuous relief workload in baseball, and lost some command of his fastball. Or perhaps hitters were finally figuring out his two-pitch repertoire as Betances finished his second go-around in the majors. In truth, there is no way to provide a specific, satisfying answer. Regardless, manager Joe Girardi is surely aware of the signs of decline that Betances showed down the stretch. With both Andrew Miller and now Aroldis Chapman in tow, Girardi should be able to ensure that every member of his high-powered trio remains well-rested. It is possible that Betances simply requires slightly decreased usage in order to hold up over the course of the whole season, and if that is case, Girardi is in position to make that happen. Despite the signs of wear, we have every reason to believe that Betances will be as dominant as ever in 2016.