For a longer (and fairly technical) answer, see "Slapped by the Invisible Hand". Essentially, a subprime mortgage had the price-will-rise-forever view baked in

For a longer (and fairly technical) answer, see "Slapped by the Invisible Hand". Essentially, a subprime mortgage had the price-will-rise-forever view baked in, as a way for the borrower to passively increase equity in the house while making very low payments. When the prices stopped rising, these borrowers went underwater fast and could not refinance when they needed to.