AMD has been faltering for quite a while now, and though recently the Radeon HD series has made a very nice comeback, times have still been tough. The purchase of ATI set them back seriously in financial terms, and the continued losses to Intel in the CPU market hasn't been any help. As a result, the CEO of AMD has opted to "take responsibility" for the decline of the tech giant and is stepping down.
Hector Ruiz also led AMD through much of their glory days in the early 2000s, so it is hard to actually place blame on a single person, but perhaps a new person in the top spot can provide a refresh on where the company is going.