I think from a simple standpoint, no citations are needed:
Fact #1) The U.S. borrows a bunch-o-money, therefore there exists a bunch of money available to lend.
Fact #2) Owners of that money would prefer a higher yield, but there does not exist enough alternative assets to put such a hugh amount of money, e.g. look at gold and oil price curves. Real estate is an option for some but it's messy and illiquid.
Fact #3) There currently exists no viable plan for the U.S. to reduce their unfunded liabilities, so what exactly are the owners of this money putting their trust in?
Safe assets are the most desirable, and in many cases the only permissible, collateral. Given increased collateral demand for financial institutions for reserves, margin, and collateralised borrowing and the potentially shrinking pool of non-Treasury "safe assets" not already on central banks' balance sheets the "price" (yield) of (on) safe assets would be expected to degrade swiftly without regard to concerns about the "safe" asset's quality.
