When Bungie left the Halo series to Microsoft, composer Martin O'Donnell stayed with Bungie and Kazuma Jinnouchi (who had previously worked on Metal Gear Solid 4: Guns of the Patriots) was hired to create music for the series. He has been with the company for seven years and did some of the music for Halo 4 and provided the entire soundtrack for Halo 5: Guardians. Most people assumed he would do Halo Infinite as well, but that doesn't seem to be the case.
Jinnouchi writes on Twitter that he is leaving 343 Industries to go freelance: "After an amazing 7-year ride, today is my last day at 343 Industries as an in-house composer. I'm becoming a freelancer and starting a new chapter of my adventure. HUGE thank you to @Halo and the community for letting me do what I love! I wouldn't be what I am without you."
There doesn't seem to be any animosity involved, but the question remains as to who will provide the soundtrack for future Halo games. It was recently rumoured that Martin O'Donnell (who composed most of the music for all of Bungie's Halo games) would be involved again, but whether this has anything to do with Jinnouchi leaving remains to be seen.