I think the biggest change was his slider was terrible last year and really good in 2014. When hitters only have to worry about 1 pitch its much easier to hit. He could get away with fastballs in the middle up because hitters had to worry about the slider.
Edit: He's also had issues with tipping his pitches especially with runners on base.
3.67 FIP with no runners on and a 5.33 FIP with guys on base last year.