  According to N-body numerical simulations, the radial pairwise velocities of
galaxies have negative skewness in the quasi-nonlinear regime. To understand
its origin, we calculate the probability distribution function of the radial
pairwise velocity using the Zel'dovich approximation, i.e., an analytical
approximation for gravitational clustering. The calculated probability
distribution function is in good agreement with the result of N-body
simulations. Thus the negative skewness originates in relative motions of
galaxies in the clustering process that the infall dominates over the
expansion.
