Why is the Fed worried about short term interest rates going negative?
Market observers worry that this wall of liquidity could push short-term interest rates into negative territory.

Fed pays interest on excess reserves (IOER) as a tool of monetary policy since late 2008.
https://www.clevelandfed.org/newsroom-and-events/publications/economic-commentary/2015-economic-commentaries/ec-201502-excess-reserves-oceans-of-cash.aspx
The &quot;wall of liquidity&quot; would increase excess reserves. Fed can pay interest on those reserves so the IOER rate does not have to go negative unless Fed decides to try that experiment as a tool of monetary policy.
Negative interest rates are the same as paying a net fee for keeping deposits or other investments at the bank or central bank. Extra central bank liquidity does not motivate banks or the central bank to charge fees. Banks charge fees to compensate for a loss or lack of net interest income. Central banks might try to make interest rates negative as a tool of monetary policy.
Repurchase Agreements with Negative Interest Rates:
https://www.newyorkfed.org/research/current_issues/ci10-5.html
Negative rates on repo mean the lender will pay interest to obtain the security collateral. This implies that investors would rather hold certain securities, which are under shortage, instead of other financial assets. One needs some case studies and/or theories which demonstrate or predict that a surplus of excess reserves somehow triggers a shortage of securities in repo markets. However if Fed imposes negative interest rates on reserves that is a different analysis.