First of all, the media is pathetic about this. As soon as the market dips, they claim to know why. Every time. Funny they never tell you ahead of time that it's about to dip.
The coronavirus is old news and the market, which is far more forward looking than the media is, had already decided it wasn't that concerned. Disruptions to supply chains aside, the mortality rate is about the same as the flu.
This dip is the market pricing in the possibility of a Sanders nomination/presidency. You'll notice it began immediately upon his decisively winning the Nevada caucus.
Never listen to the news media about their "reasons" for markets doing anything. In fact you can do quite well counter-trading anything they suggest.
The market was just too far away from the 200 daily moving average. This is just a typical pullback.