The negative correlation that has existed between U.S. Treasury bonds and broader equities over the past 30 or so years is well known, and essentially provides the case for longer duration assets to be used as a buffer against market drawdowns.
When equities sell off in a crisis, central banks often step in, buying fixed-income assets and targeting lower reserve borrowing rates to-and among-banks in an attempt to reduce short-term interest rates. Treasuries often rally as investors flee from equity volatility into high-quality bonds, further lowering and flattening the yield curve. In these environments, where short-term market shocks often lead to rapid rate cuts, the market value of long duration bonds tends to rise, giving investors substantial protection against volatile markets.