I'm still trying to figure everything out myself. Initially, it has to do with a line in the "Dominian Act" of 1867 that states that Canada was founded as two nations. New France (Quebec) and British North America (The rest of the country). After the failure of the Meech Lake accord, The Bloc Quebecois started up (A Right-Wing Seperatist party) mostly from former members of the Progressive Conservative party.
The largest argument for seperation, historically, is that seperatists believe that the province of Quebec should have exactly equal weight in the decision making process as all of the other provinces combined. In other words, British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, New Brunswick, Nova Scotia, Prince Edward Island, Newfoundland and Labrador, and the territories would combine for 1 vote, and Quebec would get 1 vote, based on the "two nations" concept of the Dominion act.
Of course, that's just a small history lesson. Expo '67 also had some seperatist sparks too. But things are always changing. I may be wrong entirely now about the current motivations for seperation.