After suffering through a 105-loss season in 2010, their second 100-loss season since the turn of the century, the Pirates have produced extended stretches of winning baseball in the two subsequent seasons. In 2011, they were 53-49 and two games out of first place on July 25 before losing 12 of their next 13 games en route to a 72-90 overall record. In 2012, they were 67-54 on Aug. 19. They were still .500 a month later but fell to 79-83 by the end of the season, their 20th straight below .500.
The Pirates' progression has coincided with the breakout of star center fielder Andrew McCutchen. McCutchen was already a three-win player per FanGraphs' WAR in each of his first two seasons, and he has matured into one of the best players in baseball in the two seasons since.
The reason the Pirates have failed to sustain their stretches of success the past two seasons has been a lack of help for McCutchen. Even with his offensive contributions, the Pirates have finished 26th and 23rd in runs scored the past two seasons. So why have the Pirates improved? It's the same reason they are on course to finally finish above .500 this year: defense.