  We use the Henriques et al. (2015) version of the Munich galaxy formation
model (L-GALAXIES) to investigate why the halo and stellar mass scales above
which galaxies are quenched are constant with redshift and coincide with the
scale where baryons are most efficiently converted into stars. This model
assumes that central galaxies are quenched by AGN feedback when hot halo gas
accretes onto a supermassive black hole. Nevertheless, we find that supernova
(SN) feedback sets both mass scales. As haloes grow above a threshold mass, SNe
can no longer eject material so their hot gas content increases, enhancing the
cooling rate onto the central galaxy, its cold gas content, its star formation
rate and the growth rate of its central black hole. Strong AGN feedback
terminates this short-lived phase by suppressing the fuel supply for star
formation. Despite strong evolution of the halo mass - temperature relation,
quenching occurs at a redshift-independent halo and stellar mass which
coincides with the mass where baryons have been converted into stars with
maximal efficiency. These regularities and coincidences are a result of the
specific parameters selected by MCMC tuning of the model to fit the observed
abundance and passive fraction of galaxies over the redshift range 0<z<3. Thus
they are required by the observed evolution of the galaxy population, at least
in the context of models of this type.
