Why Trea yield dropped to record low on Nov 24th

--$600 bill mortgate purchase plan spurred demand for Mortgages and T is highly demanded as a hedge for mortgage bonds. --As a result, yield spread for Mortgage narrowed because of strong demand; Yield narrowed to 2.97%; and Swap spread declined 30 bp to 3.2% Treasuries rallied yesterday as the Fed’s plan to purchase as much as $600 billion in mortgages spurred demand for U.S. securities as a replacement for bonds that are backed by home loans that may be repaid early. Investors holding mortgage securities often use swaps and Treasuries to guard against swings in interest rates, which can trigger changes in levels of expected mortgage refinancing and duration, a measure of the average maturity of their holdings. A swap is an agreement to exchange fixed- for variable-rate payments.