  We analyze the properties of the shallow decay segment with a sample of 53
long Swift GRBs. We show that the distributions of its characteristics are
log-normal or normal, and its isotropic X-ray energy (E_{iso,X}) is linearly
correlated with the prompt gamma-ray energy, but with a steeper photon spectrum
except for some X-ray flashes. No significant spectral evolution is observed
from this phase to the follow-up phase, and the follow-up phase is usually
consistent with the external shock models, implying that this segment likely
due to a refreshed external shock. Within this scenario, the data are generally
consistent with a roughly constant injection luminosity up to the end of this
phase $t_b$. A positive correlation between E_{iso, X} and t_b also favors this
scenario. Among the 13 bursts that have well-sampled optical light curves, 6
have an optical break around t_b and the breaks are consistent with being
achromatic. However, the other 7 bursts either do not show an optical break or
have a break at a different epoch than t_b. This raises a concern to the energy
injection scenario, suggesting that the optical and X-ray emissions may not be
the same component at least for a fraction of bursts. There are 4 significant
outliers in the sample, and this shallow decau phase in these bursts is
immediately followed by a very steep decay after t_b, which is inconsistent
with any external shock model. The optical data of these bursts evolve
independently from the X-ray data. These X-ray plateaus likely have an internal
origin and demand continuous operation of a long-term GRB central engine. We
conclude that the observed shallow decay phase likely has diverse physical
origins(Abridged).
