  Motivated by recent experiments observing a large antidamping spin-orbit
torque (SOT) on the surface of a three-dimensional topological insulator, we
investigate the origin of the current-induced SOT beyond linear-response
theory. We find that a strong antidamping SOT arises from intraband transitions
in non-linear response, and does not require interband transitions as is the
case in linear transport mechanisms. The joint effect of warping and an
in-plane magnetization generates a non-linear antidamping SOT which can exceed
the intrinsic one by several orders of magnitude, depending on warping
parameter and the position of Fermi energy, and exhibits a complex dependence
on the azimuthal angle of the magnetization. This nonlinear SOT provides an
alternative explanation of the observed giant SOT in recent experiments.
