In an answer to one of my other questions: Why does falling global bond yields signal coming deflation, the answerer states:
Whereas central banks set rates by policy, long term bond yields are determined by the market (i.e., supply-demand equilibrium). An inverted yield curve signals recession and implies rates are set too high relative to market expectations. If central banks set rates in positive territory when long term bond yields are negative, they will de facto force the yield curve to invert.
I cannot see the mechanism as to why if central banks set rates in positive territory when long term bonds yields are negative how this will cause the yield curve to invert.