  A number of gamma-ray bursts (GRBs) exhibit the late simultaneous bumps in
their optical and Xray afterglows around the jet break. Its origin is unclear.
Based on the following two facts, we suggest that this feature may sound a
transition of circum-burst environment from a free-wind medium to a homogeneous
medium. (I) The late bump followed by a steep decay is strongly reminiscent of
the afterglows of GRB 170817A, which is attributed to an off-axis observed
external-forward shock (eFS) propagating in an interstellar medium. (II)
Observations seem to feature a long shallow decay before the late optical bump,
which is different from the afterglow of GRB 170817A. In this paper, we study
the emission of an eFS propagating in a free-to-shocked wind for on/off-axis
observers, where the mass density in the shocked-wind is almost constant. The
late simultaneous bumps/plateaux in the optical and X-ray afterglows are really
found around the jet break for high-viewing-angle observers. Moreover, there is
a long plateau or shallow decay before the late bump in the theoretical
light-curves, which is formed during the eFS propagating in the free-wind. For
low-viewing-angle observers, the above bumps appear only in the situation that
the structured jet has a low characteristic angle and the deceleration radius
of the on-axis jet flow is at around or beyond the free-wind boundary. As
examples, the X-ray and optical afterglows of GRBs 120326A, 120404A, and
100814A are fitted. We find that an off-axis observed eFS in a free-to-shocked
wind can well explain the afterglows in these bursts.
