The number of head offices located in Toronto was already growing fast by the 30s, well before the rise of separatism.
One also sees that, after 1961, the number of head offices located in "other places" is also decreasing at a rate quite similar to Montreal’s. I would argue that it is because Toronto drained from Montreal but also from abroad since the metropolis of a country takes advantage of being the metropolis. (If there is ever one national financial market regulation agency, guess where it will be located? St-Jean New Brunswick? Guess again…)
(The first Montréal and Toronto post is here.)