The Trump administration was expected to invest massively into AI, and money invested into AI is effectively free money for AI company shareholders, that free money is priced in the stocks. With the change in direction, investments are expected to be less, which means less free money, and therefore less valuable stocks.
Here is the simplified version:
Let's say I have a company worth 1 billion dollars, publicly traded, 10 million stocks worth $100 each. I get promised $100M from the state (aka taxpayers), shareholders estimate there is about 50% chance I will actually get the money: $100M * 50% / 10M = $5. It means the stocks are now worth $5 more, so $105.
Now the government has changed, and gone are my chances to get that money, so the sock is back to $100.