The reason why P.K Subban got trade is because of two players, Max Pacioretty and Brandan Gallagher.
Brendan Gallagher really did not like Subban at all. PK. Subban knew that Gallagher didn't like him.
In the locker room, Pacio and Gally really didn't like him. They would go see the GM Marc Bergevin and tell him we want him out.
The only reason the Canadiens could have kept Subban is if they traded Pacioretty and Gallagher.