  We use the semi-analytic model developed by Henriques et al. (2015) to
explore the origin of star formation history diversity for galaxies that lie at
the centre of their dark matter haloes and have present-day stellar masses in
the range 5-8 $\times$ 10$^{10}$ M$_{\odot}$, similar to that of the Milky Way.
In this model, quenching is the dominant physical mechanism for introducing
scatter in the growth histories of these galaxies. We find that present-day
quiescent galaxies have a larger variety of growth histories than star-formers
since they underwent 'staggered quenching' - a term describing the correlation
between the time of quenching and present-day halo mass. While halo mass
correlates broadly with quiescence, we find that quiescence is primarily a
function of black hole mass, where galaxies quench when heating from their
active galactic nuclei becomes sufficient to offset the redshift-dependent
cooling rate. In this model, the emergence of a prominent quiescent population
is the main process that flattens the stellar mass-halo mass relation at mass
scales at or above that of the Milky Way.
