As stimulus money builds cash reserves in the banking system it should be assumed that the banks would want to reduce their reserve holdings via bond purchases. Those reserves that were previously borrowed to balance with lending are obligations that cost them interest at the overnight rate. It may be small, but it is still better to be holding bonds that pay interest than reserves that cost interest.

A surprising percentage of stimulus money was directed into savings since the pandemic stifled production of goods and services available for purchase. This caused the system to reverse its normal flow, along with pre-emptive QE actions that swapped bonds for reserves. The thing to remember is that QE is nothing more than a "swap" of assets, not actual money creation, as long as the collateral accepted is legit. It is when the Fed uses its common balance sheet with Treasury to vacuum high risk paper out of the banking system that more risk taking is encouraged.