Trump had been threatening tariffs for the campaign and mentioning them before. There wasn't that much new information that should have caused the plummet.

Also I will point out that it's more like the avoidance of information that caused some of it Nvidia's stock plunged on an announcement that went something like:

Sentence 1: we are putting tarrifs on Taiwan Sentence 2: except semiconductor related goods

It as if the market participants read sentence 1 and very few of us read sentence 2.

The EMH would assert that a casual observer like me wouldn't see the price gap between the time it took for people to read sentence 2. But it took several business days...