The most probable tea leave is that investors have priced in a certain amount of assumed government subsidy, and the Federal Reserve and Trump have taken some steps which indicate that the level of subsidy will be less than that.
For example: Suppose I think that the "true value" of S&P companies is probably about 1800, and the other ~700 of current valuation is the value of government subsidy. (Partly direct subsidy, partly the knowledge that if we see another >350 drop then interest rates can be expected to go down and stay down until things come back up). Therefore, any indication of decreased market subsidy that changes my expectations about the goverment's level of commitment or competency will see me shorting until the S&P hits 1900, and starting to buy if it falls under 1800, or until something else happens to change my mind.