Supply and Demand
When the supply of available houses is greater than the supply of buyers, appreciation may slow and prices may even fall, as happened in the early eighties and the early to mid-nineties.
If you are lucky enough to purchase a home during a slow period, you can be reasonably certain the economy will begin to show strength again. At times, real estate values may even surge drastically. In many regions of the country, this is precisely what occurred in the late eighties and nineties.