"I looked at what I think of as the food chain that led to the financial crisis, which was that you had individual consumers buying houses they couldn't afford, sold to them by realtors and property people who were competing to sell more properties at a higher price and so on. [...] I thought, hang on a second, classic economy theory tells you that a competitive marketplace is superior because competition provides a diversity of products which is good for the consumer, and it also, therefore diversifies risk. And yet, in this instance, competition has led every single one of these companies to copy each other, which had concentrated the risk. And I thought, Wow, that's interesting. That's specifically what's not supposed to happen."
More here.
Tangential question:
If it's to be expected that the housing market will periodically crash, why does it still get people investing in it?
Crashes happen periodically in just about everything: commodities, real estate, currencies, equities, bonds, etc. People invest in them because overall return is still expected to be positive.