I was an engineering manager in Mike's org. I heard he was pushed out.
Also, at GM he's reporting directly to the CEO whereas at Apple he reported to Eddy Cue who reported to Tim Cook. This move puts him one level closer to the CEO spot.
I was an engineering manager in Mike's org. I liked him on a personal level but he was thrown a lot of legacy services from other orgs. Cloud services is just not Apple's forte. The internal tools/infra feels 10 years behind Google (based on friends who've worked at both). I too heard that he was pushed out.