These types of jobs suck a lot of one's energy and personality into them, so the 'personal reasons' argument is understandable... If there are other reasons beyond that, I don't know... Also don't know if it's Akerson, and in fact I have a growing suspicion that it's not Akerson blocking Cadillac from realizing its full potential; it's the remnants of Old GM thinking... Maybe Don Butler got tired of fighting Cadillac's fight in GM's corporate politics battleground? If that's the case, then I agree with dfelt that he'll re-emerge at another automaker soon.
EDIT - For sure Don Butler isn't the only person in the World right now thinking of how much a job can suck oneself into it and lose grasp of the parts of life that are really important...