@Mogima Was asked to change sides by the former second division captain or what-not. I remember it happened around the time the protag got kidnapped.
I believe we are supposed to not know why it has reached the levels of them fighting each other, but it might also be something that I don't remember.
@Mogima@Cervanteskarp I also think that's the point of this chapter. Why he is suddenly have a change of heart is yet unknown, but knowing those guys they probably do some shady business to coerce him like kidnapping or some other shit.