Flight to safety. In uncertain times there is a flight to safety. US and foreign investors want liquidity, transparency and the safety of US capital markets and US real estate. The US economy has the largest pool of capital in the world.
The week after Brexit, US Treasury securities hit their lowest yields ever, even lower than when Alexander Hamilton was the first Treasury secretary. 10-year notes traded at 1.385%. 30-year bonds traded at 2.188%. A 30-year fixed rate mortgage is about 3.2%. Low interest rates have contributed to record high asset prices. Asset prices have grown much more than incomes in this expansion. Assets seem not necessarily over-valued (a bubble is not presently evident), however they are vulnerable to sudden shifts in sentiment in the financial markets, downdrafts.