It was probably the expansion of financial instruments like Collateralized Debt Obligations (CDOs) [0] along with shadow banking mortgage companies (like Country Wide [1]) that simply sell mortgages to banks that in turn securitize the mortgage into CDOs.
This is the correct answer, I think. The people who made the mortgages were no longer the people that owned the mortgages. The banks didn't even hold on to them, they sold them in tranches to investors. So no one in the actual industry had any incentive to do anything other than close mortgages.
People could get no down-payment mortgages, could get mortgages with bad credit, could get mortgages on houses they couldn't afford, because all the mortgage originators just fudged the paperwork. No one wanted to find a reason to not provide the mortgage.
Everyone being able to buy a house heated the market up, and as people got used to the market going up and up, it was seen as a good investment, and more people bought more expensive houses they couldn't afford, and mortgage originators did more shady things to make it happen.
A lot of these mortgages were adjustable rates that started at a really low rate, and in 3 years could shoot up to a much higher rate. An optimistic consumer wouldn't worry much about that. But when the time came, some people couldn't pay the crazy increase in their mortgage. The foreclosures coming onto the market depressed the market.
So the bubble finally popped, and someone owned a house that they bought for $750,000 with no money down, an adjustable mortgage for the whole $750,000 that started at 4% and in three years shot up to 8%, and when that happened the house could only be sold for $450,000. So they just stopped paying the mortgage and walked away. The foreclosures further depressed prices, created a very nasty cycle.
I worked in land records at the time, and the standards for the mortgage originators were just horrible. Not bothering to record mortgages in the correct town, not bothering to do a lot of things.
There are a few movies that are really interesting that described what happened, real life thrillers IMO. "Margin Call" is awesome, "The Big Short" explains what happened really well and is pretty funny at times, and "Too Big to Fail" is from the regulators point of view. It's kind of awkward but still really interesting.
I got my start as a DBA working for a company that was founded to serve as oversight for this process. We'd assign a risk label to a loan (from the originator) and then weight our aggregate risk against the documented risk of the financial instrument (a security from the "bank"). Finally, there was a group within the company to follow up with the loan servicers to ensure that they were properly servicing (erm, collecting) loans within that instrument.
It was insane how many new dimensions we'd have to create (on what seemed like a daily basis) for the variety of security and documentation types that the banks would generate.
Baby boomers paid of mortgages and use the savings to start buying houses to add to their retirement portfolio
More and more dual income housing, people can afford to spend more on a house so prices go up, then families have to have two incomes to have the same house that the previous generation had on one income
More households -- parents get divorce, now it's 2 houses needed for 2 parents and 2.4 kids
