But, of course, we have a theory. We believe that this crash is the result of the market pricing in a long overdue U.S. recession. We have no hard evidence of this, and won't until long after the recession has been in progress. But there's strong circumstantial evidence. Ask around, you'll find some, too.
Recessions start in people's heads.
Just as a market crash starts when someone, for some reason, starts to sell, recessions start when people stop buying things. Perhaps they have decided they have enough stuff. Maybe their credit card debts are getting out of hand. Or they somehow sense that they may not have the income in the future to pay debts. No one really knows.