If enough time has passed since the last crash, people tend to forget crashes may happen in the future and get careless in their dealings. Subprime mortgages weren't considered risky prior to 2008, because, if a borrower got a divorce or lost his job, or fell behind because the lender increased the rate of interest on a variable rate mortgage, it was expected the rising tide of home prices and sustained demand for homes would have allowed the borrower in trouble to sell his home for more than the mortgage balance. When the home buying market slowed, sales prices no longer were enough to pay off the mortgage balance. Banks did nothing to ease the problem. Foregoing interest rate hikes could have helped, but they plowed on. Foreclosures followed. "For sale by bank" signs on every third lawn on a block did not enhance the values of other properties on the same block the same bank was holding mortgages on. FannieMae, which had been reimbursing banks for the shortfall on foreclosure sales, was unable to keep up. The banks, with unsellable houses, resorted to sheets of plywood over the windows to protect the interior. For some reason, prospective buyers did not want to pay much for a house in that neighborhood.
As an interesting aside, in 2007, Jared Kushner bought the building at 666 5th Ave, NYC for $1.6 billion. The balance on the mortgage was $1.4 billion, and the building is now probably worth half that. Apparently, Kushner's mojo has kept his building from being foreclosed on.
Part 2. Some investment bankers had come up with a great idea of selling subprime mortgages, not one at a time, but packaged into a nice bundle which was supposedly a secure investment. They devised all sorts of criteria that had to be met in order for individual mortgages to be included in a package. These were marketed to institutions, like pension funds, looking to invest money, and naturally, there was a nice commission from each sale. The safeguarding criteria turned out to be smoke and mirrors. Some banks failed, and others were deemed too big to fail and were bailed out by the US Government. The banks explained that it was unqualified buyers falsifying their assets that induced the banks to give them loans. It is to laugh. Everybody in the loan arranging process was making money, real estate, brokers, mortgage brokers, lenders. Blindfolds were general issue. To make a sale go through, brokers sought out real estate appraisers who would put an inflated value on a property so a higher mortgage amount could be justified.
Anyway, this is my view from working in the real estate trenches at the time. There are certainly other takes on the problem.