Apparently, the Canadiens head coach had a serious conversation with the enforcer earlier this year, telling him he needs to stop taking so many unnecessary penalties. The young defenseman has been among the worst players in the league in terms of penalty differential since his NHL career started.
This same X account went on to address this theory, agreeing that the youngster could learn to pick and choose his moments better, but went on to argue that he is still a valuable asset to the rebuilding team.
The official reason for his demotion is unclear, but it seems that the team is treating it, or at least pretending to treat it, like a conditioning stint. However, this does not make much sense given the nature of the injury he is recovering from. Typically, players do not go on conditioning stints unless they have missed a significant amount of time as in multiple months.