Damn you The Verge, I hate 'heart-stopping' article titles - "The creator of 'Adventure Time' quietly stepped down as showrunner last season".
Sounds like he simply got over-worked and too stressed/exhausted to continue at the helm. Luckily Pendleton's still working away on Adventure Time and other projects, although I find this quote saddening.
Asked if he'd ever want to create another TV show, Ward responds with horror: "No, never. That sounds like a nightmare!"