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The economic term, "black swan", is something unforeseen that comes out of left field to tank the market.
In this case, the swan is orange.
Trump was the first to tout tax cuts which sent the stock market to new breaking highs. In other words, Trump promised the sky and the markets shot to the moon.
Trump also promised low and middle classes huge tax cuts. Trump initially promised $25,000 single/$50,000 married couple standard deductions. This is why many common folk voted for Trump.
But the GOP's proposed tax cuts for the rich is an abomination that will greatly cut taxes on the most wealthy while increasing taxes on many low and middle incomes. In other words, the GOP's proposed tax reform has initiated class warfare and tells the lower and middle classes to "eat cake."
Trump has manipulated the GOP between a rock and a hard place... whether their tax cut for rich oligarchs passes or not... it won't be pretty.