In an effort to boost runs in San Diego, the Padres moved in the fences before the 2013 season. The move, detailed and analyzed here by Jeff Sullivan, has resulted in little more than an increase in homers. There has been a slight decrease in doubles and triples, but that could be noise. To get more of an idea of what the Padres did with their stadium, this article has some extra information.
Here's the thing about that homer increase — it's not a uniform one. Obviously. The fences in right field were shortened, not left field. That means lefties have found a much easier time to hit dingers at Petco, as noted here:
As another example, the consequence of moving the fences uniformly closer by 5 ft would be an increase in home-run production by 13 percent. Other examples abound, but I’ll leave those for another day.
Although the fences moved in by 11 feet, they didn't move across the board. The alley was shortened, but down the line stayed the same. Alan Nathan's comment quoted above would suggest a 28.6 percent increase in home runs if the relationship is linear, but again the fence-shortening was not uniform. Comparing 2010-2012 vs. 2013-2015, as Sullivan did, there were 412 dingers in the latter period. There were 327 homers in the former period. That's an increase of 85 home runs, or a 26 percent raw increase. That's pretty close. Sullivan compared homers at Petco vs. road homers and found a 19 percent increase in homers hit at home relative to homers hit on the road; this essentially controls for the Padres having better or worse home-run hitters.
Here's the big difference: in that 2010-2012 period, Petco saw only 30 percent of its home runs from lefty hitters. From 2013-2015, 45.6 percent of Petco's homers came from lefties. In contrast, 41.3 percent of all pitches in Petco in 2010-2012 were thrown to left-handed hitters. That increased to only 44.7 percent in 2013-2015, not enough to explain the increase in homers by lefties. The fence move here had its intended effect. Lefties can hit in Petco.
The environment change has had an unfortunate effect on the Padres pitching staff. In 2015, the Padres rotation consisted mostly of James Shields, Tyson Ross, Andrew Cashner, Ian Kennedy, and Odrisamer Despaigne.
Every single one of those pitchers is right-handed.
Shields' big pitch is his changeup; consequently, he's never dealt much with platoon splits. His career wOBA platoon split is non-existent (.315 vs. L, .316 vs. R). In 2015, Shields gave up a .380 wOBA to lefties and .288 to righties.
Ross' big pitch is his slider, so he's always run a platoon split. He allowed the same wOBA to lefties in 2015 as he did in 2014, basically. However, the vast majority of Ross' innings have come with the Padres since 2013. Ditto Cashner (though his big pitch is a fastball), who also runs a platoon split but saw it at its worst in 2015. It's possible that these guys' platoon splits are just normal; they're still gathering playing time.
That leaves Kennedy and Despaigne. Kennedy is a fastball/changeup/curveball guy whose career platoon split resembles Shields' split — that is, nonexistent. He's been with the Padres since a midseason trade in 2013. He hasn't had much to say in terms of platoon splits since joining the Padres, but take a look at his home/away splits against lefties. In 2015, Kennedy gave up a 28.2 HR/FB rate against lefties at home; that was 10.9 percent away. There was a similar ratio in 2014. Shields saw a similar home vs. L / away vs. L HR/FB split in 2015 as well.
While Despaigne is still gathering playing time, he has seen a slight platoon split so far (.332 wOBA allowed vs. lefties, .312 wOBA allowed vs. righties). He has not seen the same HR/FB splits as Kennedy and Shields. Neither have Ross and Cashner.
It seems like Kennedy and Shields were disproportionately affected by what's been happening in Petco. Maybe. Looking at Shields' home run tracker, the homers he gave up were not exactly weaklings. Every homer he gave up would have been out of at least 12 parks, and he gave up homers to all sides of the field. Kennedy also gave up homers to all fields, but he gave up two lucky shots, one to lefty Adrian Gonzalez down the right field line and one to righty Howie Kendrick in center field. Both homers were homers only in Petco according to ESPN's tracker.
Kennedy will like his new home in Kansas City. Of the 31 home runs he gave up, ESPN's tracker says that maybe about nine of them would have stayed in the park. For Shields, maybe two or three would have stayed in his former home. Though not all pitchers were affected equally, having five right-handed starters in an environment that appears to be friendly to lefties can't possibly be suppressing dingers.
Another thing to consider in why Petco's dingers went up is the guy behind the plate. Though homers were up in the 2013-2015 period, 2014 saw a lower numbers of homers that was very 2010-2012-like. Rene Rivera and Yasmani Grandal were behind the plate for most of 2014. Those two are noted defensive specialists, especially in the pitch-framing department.
Baseball Prospectus released their new catcher stats recently, and Rivera ranked first among all MLB catchers in fielding runs above average and second overall to Austin Hedges. Grandal ranked 29th overall and 12th in MLB. The catching tandem in San Diego in 2014 was beyond excellent.
The Padres' primary catcher in 2013 was Nick Hundley, who ranked 399th out of 417 catchers with -15.3 fielding runs above average. In 2015, Derek Norris and Hedges handled catching duties. Norris was a poorly-rated catcher prior to 2015, but the Padres appeared to have taken on Norris and trained him in their ways. The writers at Gaslamp Ball agreed: Norris was terrible at the beginning of the season but got better at framing as the season went on.
If pitchers are confident in their catcher's ability to steal strikes on the edge, they theoretically would throw more pitches on the edge. You know, away from where most hitters like to hit dingers. This can be examined empirically, but that is another article. This one has rambled on long enough.
Norris ended up ranking 11th overall in fielding runs above average in 2015, but there was still a significant downgrade from the Rivera/Grandal combo to the Norris/Hedges combo. Norris appears to be a hard worker, and the Padres appear able to teach framing in some way. Maybe next year will be better.
There's probably some good old-fashioned luck in there too. Maybe the Padres should start a lefty sometimes.