First off, as we all know, Vogelsong has been awful lately.
The first column is his performance from when he dropped into the Giants rotation and began surprising everyone with his effectiveness (April 28, 2011, at, ironically, Pittsburgh) until August 8th of this year. The second column are his seven starts since then, starting August 13th at home:
However we slice it, he's been a whole lot of bad lately - and it's pretty much been night and day.
Some people have been saying, "Thirty innings post-'breakdown' is a small sample size, and he probably wasn't a true-talent 2.55 ERA pitcher anyway. So, perhaps there's just been a long-overdue regression, and/or some bad luck." And some numbers do seem to suggest that he is fundamentally the same pitcher he has always been, just having run into a stretch of bad luck - fielding-independent metrics seem basically to think that homie is pretty much same as he ever was, and that the 10.32 ERA has, again, just been bad luck.:
Still, an ERA ballooning out over seven whole starts to four times what it had been for the previous forty-nine, a BABIP of .483 - it does seem to warrant some further investigation.
He's been getting hit hard. That is clear.
Hmmm ... fly ball percentage about the same, line drives are slightly up (but not as up as those ERA and BABIP increases would suggest), ground balls are down, and pop-ups are way well, up - maybe he's not getting the sink he intends on his pitches?
Usually when a pitcher starts doing poorly, their K rate drops and their walk rate rises. But, since his troubles began, Vogelsong's K rate has climbed noticeably, while his walk rate has actually dropped. Strange.
Let's explore that deeper:
He is indeed getting more strikes - mostly because he is pitching more in the zone, and batters are swinging more, both inside the strike zone and out. Contact rates are down outside the zone, but slightly up in it.
Multiplying Zone%, ZSing%, and ZContact% - it looks like Vogelsong's established percentage of pitches that were in the zone and that a batters swung on and made contact with was 23.3% of all pitches thrown, but that has risen to 26.6% since his problems began. That may explain some of the rises in hits, homers, and strikeouts.
Moving on to examining his individual pitches ... first, his fastball:
(wFA/100p is a Fangraphs number that I understand to mean something like, for every 100 times the guy throws this pitch, how many positive or negative runs can it scientifically be expected to, on average, add to or subtract from his team's result (relative to a "major league average" pitch, which would be 0.00) based on the results he gets with it - getting ahead in counts or falling behind, allowing hits or getting outs, etc. Positive 1.50 is a great-results pitch, negative 1.50 is an awful one.)
Anyway, velocity on Vogelsong's fastball is slightly up (as is the case with many of his pitches), average horizontal break is a little less. He's been throwing it a lot more lately, perhaps because his two-seamer and slider have been unreliable and he's been throwing them less (see below). Results on his fastball have gone from a strong pitch to a very weak one - perhaps because he's been throwing it so much more, and hitters have been sitting on it?
The two-seam cutter:
Average velocity and break are about the same, but, somehow, it's gone from his best pitch to a haunted house slasher-flick.
Always a poor-results pitch for him, results have been even worse lately (although not as bad as the two-seamer).
He's getting less vertical break on it (perhaps explaining some of the increase in pop-ups?), and much worse results.
Tiny bit more horizontal break, tiny bit less vertical, much worse results.
I haven't had a TV for years, so, since his return, I have only watched Vogelsong pitch as an icon on the gameday or as an ant-sized man down on the field, four levels down from me. Also, I have never pitched (beyond just horsing around with friends), so I don't have as much of a visceral feel for what all these numbers indicate as I might. With that context, I find it difficult to draw any firm conclusions from all this.
And it could be that perhaps Vogelsong's problems have just been small sample size bad luck/ERA regression, and perhaps there is no reason to think that he will not have a run of starts starting with his next turn in the rotation that are similar to those we have come to enjoy from him in the past couple years.
Nonetheless, if I had to guess, the explanation I would guess for Vogelsong's recent problems may that his two-seamer started becoming less reliable in late July (a few games before his "bad period" started), leading him to throw it less, and more to pound the zone with normal fastballs with less horizontal movement- that's what I see looking over the game logs of percentage of different pitches thrown, and how useful they were. And perhaps an increase in fastballs thrown in the zone explains the increase in strikeouts, but also the big increase in balls knocked for base hits and homers.
(Just noticed that Baggs seems to have come to a similar conclusion a couple days ago: "Vogelsong continues to hit 92-93 mph with consistency. But he’s throwing more straight fastballs, and fewer pitches with sink and cut. He hasn’t had confidence in his cutter in recent starts – the pitch that broke so many bats two springs ago and got him noticed as a non-roster invitee.")