According to the Canadian census, Canada's largest cities suffered significant population declines in the 1970's (from 1971 to 1981, Montreal declined from 1.2 million to 980 000, Toronto from 712 000to 599 000, and Vancouver 426 000 to 412 000). Meanwhile, medium sized cities like Calgary, Edmonton, Winnipeg and the Toronto Area cities (York, Mississauga, etc) saw significant growth.
https://en.wikipedia.org/wiki/List_of_largest_Canadian_cities_by_census#1971
I've always understood the 20th century to be one of almost linear urbanization. What happened to Canada's urban populations in this kind of rebalancing?
In the case of Toronto, it is probably worth keeping in mind that a 1988 Canada Year Book is going to have Toronto's stats for the old City of Toronto before the 1998 amalgamation of Toronto, Etobicoke, Scarborough, North York, East York, and York into the 'Megacity.' A lot of the shift in Toronto's population stats in the 1970s would have been attached to relatively higher real estate prices and residential taxes (at the time) driving people into the inner suburbs (the soon to be amalgamated cities I just mentioned earlier) and the 'bedroom communities' of the Greater Toronto Area: Oakville, Mississauga, Brampton, Vaughn, Richmond Hill, Markham, Pickering, Ajax, Whitby, Oshawa, and others. With the implementation of Ontario's Greenbelt Act of 2005, growth of these outer burbs has been curtailed. Interestingly, relatively lower residential taxes since amalgamation has spurred a boom in new development within Toronto-proper, and it has recently passed Chicago to become the fourth largest city in North America.
All of that is to say I suspect --in the case of Toronto at least-- that it was a shift into the nearer suburbs that would fairly soon be considered part of Toronto anyway rather than an actual decline that you are seeing in your Year Book.
I cannot speak at all for Vancouver of Winnipeg, but I would say Calgary and Edmonton were experiencing a major economic boom during the 1970s as Alberta's oil industry took off. Meanwhile, the population decline in Montreal has to be tied at least in part to the migration of Anglo-Canadians from Montreal to Ontario during the 1970s and 1980s in response to the Quiet Revolution and the rise of Quebec Nationalism.
Part of the population decline in Montreal, in addition to the typical exodus to the suburbs that most North American cities experienced during that time, is related to the FLQ/sovereigntist movement in Quebec. The rise of separatism prompted a significant exodus of anglophones from Montreal to Toronto and other cities in Canada.
In addition to being made to feel unwelcome by the french nationalist movement, many companies had their corporate headquarters in Montreal but relocated to other cities in Canada, hurting the economy and employment opportunities in Montreal. There were concerns about some industries being nationalized as happened with the utility companies that were turned into Hydro Quebec.
The exodus began slowly in 1970 after the FLQ crisis (a terrorist separatist organization called the Front de Liberation du Quebec) where a cabinet minister and a British diplomat were kidnapped (the cabinet minister was killed), accelerated in 1976 when the separatist Parti Quebecois won a majority in the provincial legislature, and reached a peak in 1977 when Bill 101 was passed, imposing significant restrictions on the use of English for business in the province.
Estimates are that about 100,000 anglos left Montreal between 1976-86.
It was around this time that Toronto passed Montreal as the biggest city in Canada.