The uncertainty over Brexit’s impacts carried through the weekend and is the most important factor causing a market correction because the market had already priced in a “stay” vote.
The massive decrease in the value of the pound sterling (3%, large for a currency) to $1.31, a level not seen since 1985, and the ensuing fall in financial stocks in the US (as well as in the EU) pulled markets lower, mainly because banks were already positioned for a hike in interest rates which will now not materialize.
The global uncertainty affected small capitalization stocks more negatively (the NASDAQ fell more than the Dow), although that effect was offset by the fact that international tensions do little to sway domestic share prices other than a general correlation. The Utilities SPDR ETF (XLU) reflects this as it was in fact up 0.87% today.