It's not just Toronto, but also cities outside of the GTA (like Guelph, Kitchener/Waterloo, Cambridge, etc.).  My theory is that people are being priced out of Toronto and looking for cities within a commutable distance (although I would never consider commuting from my place in Guelph!).  As the Toronto prices increases, the neighbouring cities increase due to the spillover effect.

It's insanity in the market right now -- I bought in Guelph in Sept 2015 for $390K, and I'd be lucky to buy the same home now for $550K.