Add to that investor pessimism driving bond yields down or inverting them, a trade war, bad global growth numbers, and investors sucking every last penny out of the stock market while there is still time. That's all you need for a downturn.

That said I'm not predicting a huge downturn. As long as there is some political correction back towards the center or left, which there usually is after a downturn, they'll be driving the Dow to 35k+ in no time. History repeats itself.