I think that the main problem is that the recent annoucement for duty taxe (steel, wood, farm products, etc.) on many non US product have help to decline the dollars. Since then, probably many investors have sendback their money in euro or canadian dollars. Many big american finanacial have put a warning againt these duty taxe, and now we observe the result on the markets. Less money, lower indices!
In the 90 years, the fuel of the markets was that the free market across the USA and with the outside world, now too many restrictions.