That would be the massive amount of QE & money injected into the system. I doubt next recession it will be so simple to sell the bonds we need to to run a massive QU campaign again
[0] Fed total assets from FRED: https://fred.stlouisfed.org/series/WALCL
The fed also expanded the balance sheet by selling long term bonds, which offered relatively lower rates in comparison to short term notes and bills, this provided the cash infusion needed for QE to stimulate the economy.