They’re only the floor because they’re seen as risk free. “Why lend someone money for less than what the US government is offering you? The government is always gonna pay you back, after all”. If that mentality changes then treasury bonds will no longer be the floor, because you’d rather lend the money to someone else than the US government.

Not that this isn’t disastrous for the US. Increased taxes, cuts to medicare/medicaid/military, a government default, or a mix of all three are an inevitability. The US government can probably keep paying interest payment costs with more debt for a while, but not forever. These movements in the bond market takes us closer to the end of the USA’s debt spree.