2025MNRAS.541..573D

In part I of this series, we discussed the variation of star formation histories (SFHs) across the specific star formation rate - stellar mass plane (sSFR-M$_{\star }$) using the Deep Extragalactic VIsible Legacy Survey (DEVILS). Here, we explore the physical mechanisms that are likely driving these observational trends, by comparing the properties of galaxies with common recent SFH shapes. Overall, we find that the processes shaping the movement of galaxies through the sSFR-M$_{\star }$ plane can be largely split into two stellar mass regimes, bounded by the minimum SFR dispersion ($\sigma _{\rm SFR}$) point. At lower stellar masses, we find that large $\sigma _{\rm SFR}$ values are likely observed due to a combination of stochastic star formation processes and a large variety in absolute sSFR values, but relatively constant/flat SFHs. While at higher stellar masses we see strong observational evidence that active galactic nuclei (AGNs) are associated with rapidly declining SFHs, and that these galaxies reside in the high $\sigma _{\rm SFR}$ region of the plane. As such, we suggest that AGN feedback, leading to galaxy quenching, is the primary driver of the high $\sigma _{\rm SFR}$ values. These results are consistent with previous theoretical interpretations of the $\sigma _{\rm SFR}$-M$_{\star }$ relation.