I don't live in the US, and my understanding of how the electoral system works out there is not fantastic, but it would also seem like there's some impact from

I don't live in the US, and my understanding of how the electoral system works out there is not fantastic, but it would also seem like there's some impact from the existential dread of democrats taking majorities in your mid-term elections. A lot of the run up since the end of last year was based on Trump's new tax plans, which the left were very much against, so it makes sense that measures would be taken to roll that back were they to be in a position to do so - and I'd expect that people are nervous because (unpopular as Trump is in polls) no-one knows how that's going to go. It seems (like here in the UK to be fair) that it's going to be a toss of a coin as to which extreme your economy is swung towards in the next election. Also, new money is entering the financial markets all the time that doesn't have the past experience of the ups-and-downs. People can be well read on the topic and still not have a steady hand when it comes to the first turbulence event they experience with their savings, so there will always be (or at least, has always been) a proportion of panic sellers in the market at any one time. (edit: This reads like I'm very right-wing. I'm really not, all I mean is that the market has gone up because of the big giveaway that might be snatched back again soon.)