  The power-law decay of the X-ray emission of GRB afterglows 050319, 050401,
050607, 050713A, 050802 and 050922C exhibits a steepening at about 1--4 hours
after the burst which, surprisingly, is not accompanied by a break in the
optical emission. If it is assumed that both the optical and X-ray afterglows
arise from the same outflow then, in the framework of the standard forward
shock model, the chromaticity of the X-ray light-curve breaks indicates that
they do not arise solely from a mechanism related to the outflow dynamics (e.g.
energy injection) or the angular distribution of the blast-wave kinetic energy
(structured outflows or jets). The lack of a spectral evolution accompanying
the X-ray light-curve breaks shows that these breaks do not arise from the
passage of a spectral break (e.g. the cooling frequency) either. Under these
circumstances, the decoupling of the X-ray and optical decays requires that the
microphysical parameters for the electron and magnetic energies in the forward
shock evolve in time, whether the X-ray afterglow is synchrotron or
inverse-Compton emission. For a steady evolution of these parameters with the
Lorentz factor of the forward shock and an X-ray light-curve break arising from
cessation of energy injection into the blast-wave, the optical and X-ray
properties of the above six Swift afterglows require a circumburst medium with
a r^{-2} radial stratification, as expected for a massive star origin for long
GRBs. Alternatively, the chromatic X-ray light-curve breaks may indicate that
the optical and X-ray emissions arise from different outflows. Neither feature
(evolution of microphysical parameters or the different origin of the optical
and X-ray emissions) were clearly required by pre-Swift afterglows.
