Actually QR has free reign as to where they want to fly to . But with YYZ in political turmoil with EK/EY, it was a wise move by QR to fly into YUL instead.

I suspect AC would provide feeder traffic to QR flights to YUL as QR already has agreements with other *A partners (though itself is not a *A member) from many other cities ie IAD with UA, FRA with LH.