Well whatever it is, Pike and the radioactive accident had something specifically to do with M’Benga stepping down. I wouldn’t call it demotion necessarily as there are reasons that someone could willingly step away from the position of Chief and still hold their rank. Either way, during the Quality of Mercy episode M’Benga was still CMO under Captain Pike.
With how M’Benga holds himself and takes the quality of his patients personally, and becomes heavily emotionally involved in his work, I think he stepped down willingly because he no longer believed himself to be capable of providing the support necessary to a Captain. I think maybe he worked on Pike and tried to save him but when Pike was too far gone that M’Benga became a bit despondent. We know the two are friendly and decently close so maybe between being unable to save his daughter and being unable to save his Captain he just felt he wasn’t capable of doing his best anymore. Still wanted to help and assist and thought of the Enterprise as home, and wanting to stay near his family in the crew, but didn’t want to carry the weight of that responsibility anymore.