It is also because the USA is the reserve currency of the world with open capital markets.
Savers of the world (including countries like Germany and China who have excess savings due to constrained consumer demand) dump their savings into US assets such as stocks.
This leads to asset bubbles and an uncompetitively high US dollar.
Countries like China, Germany, Taiwan, etc. have competitive exports because they have direct and indirect subsidies to their manufacturing sectors at the expense of their household sector.
Some of these subsidies include a weak currency relative to their economy, weakened labour laws, preferential interest rates, capital controls, labour movement restrictions, etc.
China uses all of these. Germany primarily used the Hartz "reforms" which basically decoupled wage growth from productivity and GDP growth.
The reduces the household share of national income and they cannot afford to consume the production of their manufacturing sector and therefore the excess production must be exported.
Perhaps the stocks were massively overvalued and any negative news was going to start this sell off regardless of its actual impact?
That is my theory anyway.