Well, the stock market had been rising since the election, on the strength of his promise to cut taxes and regulation, and increase spending.
However, the markets were assuming he was rational enough to introduce change gradually. Abrupt changes are disruptive and can cause severe problems to markets and the economy in the short run, and even in the medium term if they are disruptive enough.
Now, the craziness of Trump's handling of immigration-- without any phase-in period, without thought of the consequences for people in flight, without any consideration of the ramifications from friendly countries, without thought to impacted families, with miscommunication to the agencies (particularly for green card holders); without any plan for doing this additional vetting; without all the leadership in place to manage these new programs-- well, this makes a lot of people wonder whether he has much of a clue about the economy, how large organizations run, and business in general (outside of the peculiarities of his own real estate development business). It's okay to have limited knowledge (hey, so did Ronald Reagan); but you at least need to surround yourself with knowledgable people, and LISTEN to them. Otherwise, there's chaos. And the markets HATE chaos.