I suspect compensation was an issue. They don't have a lot of money to throw around right now, and they won't for the foreseeable future.
I think money was the major issue IMO. Keller is a specialist.... Regular engineers can handle revisions and optimizations (for the next-gen Zen+), but they definitely needed someone like Keller to design the brand new architecture.
You can see what "regular" engineers managed to design with the clean sheet Bulldozer. Considering Keller stayed 3 years versus the 1 year he worked for AMD previously, I'd imagine he left on good terms.
Zen is taping out now -- so the architecture is done. It would have been awesome if he stayed on for Zen+, but it's probably not interesting to him.... He likes clean sheet approaches versus revised products.