This change was the reversal of central bank policy. The era of stimulus and artificial support of various markets, including stocks, is beginning to fade away as the Federal Reserve pursues policy tightening, including higher interest rates and larger cuts to its balance sheet.
I warned of this change under new Chairman Jerome Powell at the beginning of 2018 in my article 'New Fed Chairman Will Trigger Stock Market Crash In 2018'. The crash had a false start in February/March, as stocks were saved by massive corporate buybacks through the 2nd and 3rd quarters. However, as interest rates edged higher and Trump's tax cut cash ran thin, corporate stock buybacks began to dwindle in the final quarter of the year.
As I predicted in September in my article 'The Everything Bubble: When Will It Finally Crash?', the crash accelerated in December, as the Fed raised interest rates to their neutral rate of inflation and increased balance sheet cuts to $50 billion per month. In 2019, this crash will continue as the fed resumes cuts once again in mid-January.
It is important to note that when we speak of a crash in alternative economic circles, we are not only talking about stock markets. Mainstream economists often claim that stocks are a predictive indicator for the future health of the wider economy. This is incorrect. Stocks are actually a trailing indicator; they tend to crash well after all other fundamentals have started to decline.