Historically the Three D's of housing that forced homeowners to sell, or into foreclosure, were Death, Divorce, or Disease.
I've seen this revision a few times recently ...
“If you sell in this market, it’s usually one of the three D’s: death, divorce or debt.”Paul Brennan, regional director for the Hamptons at Elliman, Bloomberg, Jan 7, 2009 (hat tip Rick)
Really there are four D's right now.