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I think its just pure fear selling off way too much. The stock market only appeared to be on a huge bull run, the amount of money in the system doubled since 2008: https://fred.stlouisfed.org/series/MABMM301USM189S

Since 2008 money means something different. The fed will keep releasing liquid funds for banks to buy up all the cheap stock. Or the US government might even give away money to keep consumer spending going. Either way it will expand the money supply enough until the SP500 goes back to 3000 even if 3000 equals 1500 in 2019 money.