  Context: Early-type galaxies (ETGs) are divided into slow and fast rotators
(FRs and SRs) according to the degree of ordered rotation of their stellar
populations. Cosmological hydrodynamical simulations indicate that galaxies
form as FRs before their rotational support decreases, usually because of
mergers.
  Aims: We aimed to investigate this process observationally for galaxies
outside of clusters.
  Methods: We made use of the fact that different merger types leave different
traces that have different lifetimes. We statistically analyzed multiple
characteristics of galaxies that are expected to be influenced by mergers, such
as tidal features, kinematically distinct cores, and stellar ages. They were
taken from the MATLAS and ATLAS$^\mathrm{3D}$ databases. Through multilinear
regression we identified the quantities that, at a fixed mass and environmental
density of the galaxy, significantly correlate with a measure of the ordered
rotation of the galaxy, $\lambda_{R_e}^N$.
  Results: We found a negative correlation of the rotational support with the
occurrence of tidal disturbances and kinematic substructures, and a positive
correlation with metallicity and metallicity gradients. For massive galaxies,
the rotational support correlates negatively with the abundance of alpha
elements, and for the galaxies in low-density environments, it correlates
negatively with the central photometric cuspiness. These and additional
literature observational constraints are explained the easiest if the mergers
that decreased the rotational support of ETGs were typically minor, wet, and
happening at $z\approx 2$. They did not form the currently observed tidal
features. The observed frequency of tidal features implies a merging rate of
0.07-0.2 per Gyr. This is insufficient to explain the observed growth of the
radii of ETGs with redshift by mergers.
