Why has the S&P 500's amplification factor turned negative after more than a decade of being virtually constant?
It didn't happen all at once. It has happened in stages, coinciding with significant changes in the Federal Reserve's monetary policies in response to the coronavirus recession. Those actions have created today's upside down market environment, where stock prices keep rising even though market fundamentals, the things that typically drive changes in stock prices, have been crashing.
The trading week ending on 15 May 2020 provided some insight into why that is, which saw the trajectory