... bubbles in land prices popping.
http://www.kansascity.com/mld/kansascity/business/14981787.htm
Of course, if a builder's stock goes belly up, it mainly affects investors in that stock. However, if a large number of builder's stocks go belly up it could have a negative effect on the general economy. Interestingly enough, this could put an upward pressure on housing prices, since fewer builders would likely translate into fewer housing starts, leading to imbalance on the supply side of the market. Possibly scenario: housing prices were on decline, leading to at least hiss, if not pop, of the bubble balloon... then, several builders went under and balloon starts to re-inflate.