You guys bring up a great point that maybe it's as simple as he's had enough???
Something to think about... The model that all of us grew up with was you tried to work your way up through the lower ranks... hopefully get noticed... eventually make it to cup... have success for as long as you were physically able, or until you no longer could get a competitive ride, and then just drifted away. (Sad, but true for a majority of cup drivers.) I'm thinking that prior to the 90's, and maybe even prior to 2000, few drivers, even those who had run for 10+ years, really made enough money to retire comfortably, and if they did, maybe didn't invest in retirement, so they kept grinding it out... maybe picked up a Busch ride... or a truck ride... and then, they were just no longer part of the sport.
With the type of $'s that a top cup driver can make now in a handful of years, that old model is no more. Maybe we'll see others do the same thing, and maybe we've already seen it on a slightly different scale with Gordon and Stewart retiring when many felt that they should still have some good years left.
Still seems surprising that somebody would walk away while on top of their profession, and in what I think most would agree is the best rides in the garage.