The mortgage mess in the US is the result of deregulation. Starting in Reagan's day and continuing to the present time, the regulations that dated to the Great Depression of the thirties, and which prevented a recurrence, were dismantled at the request of the financial industry. They took advantage of the newly created loopholes to drive a fleet of Brinks trucks through. When the pyramid collapsed they got a bailout from the government. There was no long term planning at all. Hell, I knew the whole thing went against hundreds of years of banking experience from the start. I thought the government, banks, and regulators had some clever new way of preventing all those mortgages from defaulting. Turned out they didn't. In fact the once you get looking into it, the mess just gets worse and worse, and filled with fraud at every turn.
The Big Short is a good book to start with. There are lots of web sites that cover it. There is one called Market Ticker that was started in 2007 to document the whole thing in real time but there are others.