It's widely speculated that he's been a locker room cancer for years now, dating back to before the Bubble season. Bruce Boudreau and Brad Richardson have both publicly commented that Miller and Pettersson have had a toxic relationship for a long time and it's clearly boiling to a head. It was thought by fans that there was a locker room divide between Horvat and Miller, so by trading the former you were solving the issue, but it's pretty clear the real issue is and always has been Miller.
Management has moved out several key veterans over the years who were most likely keeping things from truly boiling over (Tanev, Markstrom, Toffoli, Schenn, Cole, Zadorov etc.) With nobody around to keep Miller in check, as "the emotional leader" he's been put in a position to drive the room by Tocchet but it appears he has also soured that relationship to the point where they've publicly displayed signs of it. His whole LOA thing was framed by the Canucks as a "mental health reset" but it's been clear to anyone paying attention that Miller overstepped to someone in management about being benched despite Pettersson's mediocre performance at a higher cap hit.
In short - great player when he's on, disaster when he's not. He needs to be in a controlled environment, playing under someone like MacKinnon who is a similar fiery-type leader but would not put up with any of Miller's bullshit. That's where he will truly thrive. The Canucks simply don't work in that type of an environment, and it's been this way since the Benning years.