Market Crashes are not that simple. More often than not they are a form of liquidity crisis on debt instruments that are poorly designed and lack the advertised returns after long term investment. Typically these bonds have low security collateral and high risk. In 2008 this was related to the subprime mortgage bubble, the coming bubble is sometimes referred to as the everything bubble. This will probably be escalated by the Trump Tax Cuts as many in the middle class ended up paying more in the last year than in previous years due to punitive removal of certain deductions designed to punish blue states.