I figure he just wants to go and specialize, and expand his research opportunities. It's not that unusual in the medical field or other fields with people wanting to pursue a specialization within their profession. McCoy was more senior and was made CMO in his absence and he had to serve under McCoy. Generally speaking, that's not a slap in the face. It's called chain of command and it exists for a reason. None of those reasons are set up as a slight towards M'Benga or Chapel.