Veteran rotation failure
Coming in to the season, Ian Kennedy, Brandon McCarthy and Trevor Cahill were expected to anchor the rotation and help give the D-backs a chance to win, almost every day. That's not too unrealistic, as their career ERA+ through 2012 were 109, 108 and 107 respectively: while not elite, apparently solid enough. This year, however, none of them even reached average, coming in at 73, 84 and 96. Over their combined 68 starts, they went 15-29 with an ERA of 4.59 - as a yardstick, Micah Owings' career ERA for Arizona was 4.69. In total, the trio were sub-replacement level (-1.0 bWAR), while costing the Diamondbacks over $15 million.
Things came to a particular head for our rotation during a hellish streak from May 30 through to July 2. That included a streak of 24 consecutive contests without a win by any of our starting pitchers, and over the entire 31-game period, they went 3-14 with a 5.64 ERA. The bizarre thing is, even as the Diamondbacks were 12-19 over the stretch, our lead in the NL West was exactly the same at the end as the beginning: 1.5 games. However, this mass demonstration of futility by just about the entire National League West, allowed the Dodgers back into the race, when mere competence would probably have been enough to bury them.
The meltdowns of Kubel and Montero
Jason Kubel's 2012 season was respectable enough. If his range in left-field was limited (shall we say), he showed a decent arm, and had enough power at the plate to make up for it. His 1.6 fWAR was brilliant value for $7.5 million, but it wasn't a disaster either, and something similar in 2013 would have been acceptable. But it didn't happen. He didn't hit for average, his power evaporated, his defense was awful and (admittedly with some help from Matt Williams), so was his base-running. It was an implosion in spectacular style, delivering the worst season in Diamondbacks history by fWAR, at -1.7.
But an even bigger drop-off was Miguel Montero, who went from 4.6 fWAR last season, to a mere 0.9 this. I have a theory that the more adverts players do, the worse they play. Miggy had as many three-run homers, like in the Taco Bell advert, as Matt Davidson. Whom you don't see advertising crappy fast-food. Yeah, I'm bitter. After back to back years of 121 and 123 OPS+. Montero slumped to 83, and you can add an increase in wild pitches/passed balls allowed, from 43 to 59 in 183 fewer innings. Between them, Kubel and Montero were worth seven wins fewer than last season, basically representing the difference in our offensive output (28.2 fWAR to 22.0)
Blown saves, and all that entails
It's hard to tell if the D-backs bullpen were good, bad or mediocre this year. Whatever your opinion, you can probably find a metric to support it. Good? They led the majors in shutdown appearances (those which improved WP by 6% or more), with 167. Bad? The 29 blown-saves tied for the lead in the majors too. And if you want mediocrity, their overall ERA of 3.52 was almost exactly on the National League bullpen average of 3.50. As sabermetrician Benjamin Disraeli allegedly said, "There are lies, damned lies, and relief pitcher statistics."
We received a harsh lesson this year in the volatility of bullpen arms, as it seemed for a while that whoever we anointed closer, immediately sucked. But it seemed our bullpen was always pitching with no room for error. The average leverage index - a measure of the importance of the at-bat - they saw was 1.30, the highest in the league - and the second-placed Marlins were as close to 13th spot as they were to us! Part of the problem is that the most reliable relievers weren't getting the highest-leverage work. The pLI for Heath Bell, for example, was 1.62; Will Harris 0.85. Brad Ziegler being locked in to a ninth-inning role for half the year, hardly seemed optimal either.
Adventures on the basepaths
We had the worst stolen-base rate in the majors, at 60%, took extra-bases less often than average, and grounded into more double-plays than any team in baseball this year. Yeah, I think it's safe to say that getting on base was only the start of the entertainment as far at the Diamondbacks were concerned. There are a few players on the team whom you'd think have the raw tools to be a genuine threat to steal, but it's significant that Paul Goldschmidt, certainly not the quickest player on the team, had the most SB. I think there's definitely a coaching issue, when Gerardo Parra can go from 15-1 on the basepaths, to 15-9 last year, and now 10-10 this season.
Certainly, injuries played a part, though I don't think as much as Kevin Towers and Kirk Gibson have suggested of late. Aaron Hill's missed two months is the most obvious example, and pro-rating his fWAR, maybe cost us about 1.5 wins, Cody Ross's various issues perhaps another one, and it'd have been nice to get a full season out of Matt Reynolds. But some of the other ailments realistically didn't hamper the team that much, if at all: for instance, the loss of Adam Eaton opened the door for A.J. Pollock, who was worth over four fWAR more than Eaton through the year. And our best pitchers - Corbin and Miley as starters, Ziegler in relief - were entirely healthy all year.
Unless you are incredibly lucky, injuries are inevitably going to be part of every season and there's no particular evidence that the Diamondbacks were hurt worse in this department than their rivals. Few teams get to see their expected Opening Day line-up take the field on a regular basis for long, and if that's required as a condition for success, you're almost certainly going to be disappointed.
Losing by the long-ball
If it seems like we spent a lot of time watching the opposition trot around the base-paths, that's because they did. Starters or relievers, it seems that everyone on the team gave up a lot of home-runs: the final tally for the season by Diamondbacks pitchers was 176, most in the National League and 21 more - an entire Andrew McCutchen worth! - than last year. The odd thing is, we weren't particularly a fly-ball oriented staff - our GB/FB rate of 0.87 was fractionally higher than league average. However, our HR/FB rate of 8.4% was tied for the highest in the league; hopefully, that will regress a bit next season, though part of it may be playing at Chase
You might be quite surprised to learn that, despite the struggles of both our veteran starters and at times, the bullpen, we allowed almost exactly the same number of runs as last year: up a mere seven, from 688 to 695. A bigger impact on our overall results came about from the drop in run production, which decreased from 734 to 685, a loss of 49 runs, even as Goldschmidt had one of the best seasons in the past decade by a Diamondbacks hitter. But he was the only player to reach 400 PAs with an OPS+ above 105 - last year, we had five of them.
We already mentioned some causes above: Kubel and Montero were much less productive; Hill lost time due to injury; and Prado was not as productive as Upton (OPS+ for Upton in 2013 = 122; Prado = 105). The lost OPS came almost entirely from a decline in slugging: our BA was exactly the same (.259), and our OBP was five points off (.328 to .323). But Arizona's SLG dropped 27 points from .418 to .391, and that was all down to fewer home-runs. The D-backs hit only 130 this year, down 35 on last season - combine that with the increase in homers allowed, mentioned above, and you've got a fairly good idea why the team came up short.