If we begin at the most basic, a price change is always a question of demand or supply, or both. Demand, as you say, has been growing in the GTA, but the economy has been growing at 4% to 5%. It's being fuelled by immigration and job creation. That creates a very basic demand for more housing. Supply has been growing but has not kept up with that demand, and so there's a natural tendency for prices to rise. Those are fundamentals.
However, there's no fundamental story that I could tell that could justify price rises of 20% or 30%, so without being specific about just how much of it is due to speculation, it's obvious to me that a growing amount is due to speculative behaviour, which means people buying housing not to live in but to flip, etc., for investment purposes.
That of course is a more risky phase of any cycle. It means that it's time to remind people that house prices can go down as well as up and that they should be doing their own risk assessments, fundamentally asking, for example, “Why am I buying this house”, and “Could I withstand a 10% correction in prices?” Many ordinary people could; they would just continue to pay their mortgage and live there. The speculators, however, would not be able to do that, and so it's financially risky.