Quote from kjkent1:
The problem was simply that lenders permitted borrowers to leverage their homes at 100%+ of appraised value. This allowed people to purchase a home without risk.
When a person makes a risk-free investment, there's no reason to hold the position when it's under pressure -- so, the person just walks away and the market crashes.
Exactly what happened with stocks prior to the Great Depression.
Quote from nutmeg:
People don't walk away from a negative equity home, it seems pointless, they have to live somewhere, they walked away from the payments they can't make.