wrote an excellent piece detailing it. Sullivan ultimately suggests that Axford had a tremendous run of good luck followed by smaller run of bad luck. That's certainly possible, but I don't see much in the numbers that suggest he was a victim of outright bad luck; the next step is to look at his stuff.
For most of his career, he has been a fastball-curveball guy who's occasionally fiddled with a slider. During his first full season in 2010, he threw the slider nearly as much as his curve (16.2% to 18.8%), but in 2011-12 he cut down on the slider usage, throwing it less than 10% of the time while increasing his fastball usage and using the curve as his primary breaking ball. 2011 was undoubtedly his best season, and the curve was a huge weapon for him. He threw it for a strike, and it drew it's share of whiffs.
With his curve working so well, hitters couldn't sit on his fastball. Which is important, because Major League hitters can hit fastballs pretty well, even really fast ones. Now let's look at 2012.
You'll notice that his curve was far less effective. He couldn't throw it for strikes as often as he did the year prior, as it's strike percentage went from 55.36% to 45.88%. Hitters were more likely to lay off his curve and sit fastball, which means harder contact and more balls flying over walls. It is at this point that I will mention that he's more or less pushed his lonely slider aside.
"Grandpa, I'm cold."
It's not like he never threw it, but he didn't use the slider often enough to make much of an impact. The start to 2013 was more of the same. By digging through his game logs, I determined that he used his frisbee once in his first three appearances, all three of them being nightmare outings.. In those same outings, his fastball and curveball were consistently up in the zone, where the hitters feast. I don't know if he was overcompensating to make sure his curveball was a strike, but the results weren't good.
Starting with his fourth appearance, Axford had made a noticeable change; more sliders. The results in that particular outing weren't good, but the next six outings were.
This all comes with an enormous disclaimer of small sample size and whatnot, but I think this still paints something of a picture as to the adjustments Axford has made in the first few weeks of the season. Eventually hitters will probably stop chasing the slider, but by then it's possible he will have fixed his curveball or made other adjustments. If nothing else, he's at least focused on a third pitch to keep hitters off-balance, making our proclamation of his death maybe a little premature.
PITCH f/x data courtesy of Brooks Baseball and Texas Leaguers