You can put all the reasons aside for this one, the simplest: Traditional macroeconomics warns that you don't risk overheating an already hot economy by cutting taxes, since the downsides are higher inflation — which means higher interest rates — and even more government borrowing.
Politico: "A global market sell-off that began Friday continued into Monday with the Dow Jones Industrial Average dropping more than 1,500 in afternoon trading."
Is there any upside? You bet.
"This is a risk that the president clearly set himself up for," said Charles Gabriel of Capital Alpha Partners. "Until now, Trump’s had kind of a free ride in this market and taken so much credit for it…. Volatility is back."
If Trump caused the stock market to go up — which he didn't — then he also caused it to go down — which he did, actually, by pushing for idiotic tax cuts.