If Orton's leaving was reallly a choice, I'd go with the disagreement over ATI's future products.
He was the head of an independent company AMD is in the process of decapitating and cannibalizing.
ATI as an independent company was willing to take some design risks.
AMD is increasingly risk-averse, and if it's going to bet the farm on a product, it's not going to be a GPU.
I doubt ATI's recent performance has proven palatable for AMD, not at the price paid.
If AMD's going to have delays and lackluster products, it may as well be delays and lackluster products directed by AMD's core executives.
I don't see why any head of ATI would be surprised to be bitten off after courting the big green mantis.
How does the GPU market look, now that one of the two major high-end providers is an increasingly conservative x86 manufacturer?