It’s purely psychological. The “housing market” is really the “real estate” market which accounts for a lot of money. There is also a lot of related industries which is affected if the real estate market goes down. First is the home builders or construction industry (contractors), then there is the materials suppliers, then the mortgage industry which makes loans to buyers and then there’s the real estate agents, title companies, escrow companies, home insurance companies, furniture companies – all their jobs hang on how well the housing market is doing. They were in fear for the past 2 years when the housing or real estate market peaked and began sliding down. Investors in the stock market finally decided that there is too much fear and decided to sell their stocks. Selling causes the stock market to drop.