  Early-type galaxies (ETGs) possess total density profiles close to
isothermal, which can lead to non-Gaussian line-of-sight velocity dispersion
(LOSVD) under anisotropic stellar orbits. However, recent observations of local
ETGs in the MASSIVE Survey reveal outer kinematic structures at $1.5
R_{\mathrm{eff}}$ (effective radius) that are inconsistent with fixed
isothermal density profiles; the authors proposed varying density profiles as
an explanation. We aim to verify this conjecture and understand the influence
of stellar assembly on these kinematic features through mock ETGs in
IllustrisTNG. We create mock Integral-Field-Unit observations to extract
projected stellar kinematic features for 207 ETGs with stellar mass
$M_{\ast}\geqslant 10^{11} \mathrm{M_{\odot}}$ in TNG100-1. The mock
observations reproduce the key outer ($1.5R_{\mathrm{eff}}$) kinematic
structures in the MASSIVE ETGs, including the puzzling positive correlation
between velocity dispersion profile outer slope $\gamma_{\mathrm{outer}}$ and
the kurtosis $h_{4}$'s gradient. We find that $h_{4}$ is uncorrelated with
stellar orbital anisotropy beyond $R_{\mathrm{eff}}$; instead we find that the
variations in $\gamma_{\mathrm{outer}}$ and outer $h_{4}$ (a good proxy for
$h_{4}$ gradient) are both driven by variations of the density profile at the
outskirts across different ETGs. These findings corroborate the proposed
conjecture and rule out velocity anisotropy as the origin of non-Gaussian outer
kinematic structure in ETGs. We also find that the outer kurtosis and
anisotropy correlate with different stellar assembly components, with the
former related to minor mergers or flyby interactions while the latter is
mainly driven by major mergers, suggesting distinct stellar assembly origins
that decorrelates the two quantities.
