  We present a process that accounts for the steep-decline-and-plateau phase of
the Swift-XRT light curves, vexing features of GRB phenomenology. This process
is an integral part of the "supercritical pile" GRB model, proposed a few years
ago to provide an account for the conversion of the GRB kinetic energy into
radiation with a spectral peak at $E_{\rm pk} \sim m_ec^2$. We compute the
evolution of the relativistic blast wave (RBW) Lorentz factor $\Gamma$ to show
that the radiation--reaction force due to the GRB emission can produce an
abrupt, small ($\sim 25%$) decrease in $\Gamma$ at a radius which is smaller
(depending on conditions) than the deceleration radius $R_D$. Because of this
reduction, the kinematic criticality criterion of the "supercritical pile" is
no longer fulfilled. Transfer of the proton energy into electrons ceases, and
the GRB enters abruptly the afterglow phase at a luminosity smaller by $\sim
m_p/m_e$ than that of the prompt emission. If the radius at which this
slow-down occurs is significantly smaller than $R_D$, the RBW internal energy
continues to drive the RBW expansion at a constant (new) $\Gamma$, and its
X-ray luminosity remains constant until $R_D$ is reached, at which point it
resumes its more conventional decay, thereby completing the "unexpected" XRT
light curve phase. If this transition occurs at $R \simeq R_D$, the steep
decline is followed by a flux decrease instead of a "plateau", consistent with
the conventional afterglow declines. Besides providing an account of these
peculiarities, the model suggests that the afterglow phase may in fact begin
before the RBW reaches $R \simeq R_D$, thus introducing novel insights into the
GRB phenomenology.
