I would trace it back to the early 2010s when Professors at University of Toronto (Geoffrey E. Hinton in particular) came up with many of the advances in training deep neural networks. That, along with the hype of deep learning, began a snowball of activity that put Toronto on the radar in a significant way.

I remember reading the below article. Prof. Hinton can probably claim quite a bit of credit for his city's successes. http://www.nytimes.com/2012/11/24/science/scientists-see-adv...