We present a detailed investigation, based on ensemble density functional theory simulations, of the microscopic mechanisms that accompany the sliding of grain boundaries in aluminum, a typical ductile metal. We find a variety of sliding behaviors, including coupling to migration, that depend not only on the delocalized character of the metallic bonding, but also on the boundary geometry, the local order, and the presence of defects. While our previous results showed that sliding in germanium is controlled by local stick-slip events involving rebonding of a few atoms, we find that in aluminum larger numbers of atoms act in concert over extended areas, ultimately limited by boundary defects.