Low temperature (T<150 K) RHEED oscillations during the growth of ultrathin films suggest the presence of substantial adatom mobility. In most systems thermal diffusion can not account for the observed oscillations, and the origin of the oscillations is an open question. Experiments on Ag/Si(111) at 150 K demonstrate the absence of thermal diffusion due to the observed scaling in the RHEED intensity for different flux rates. We have performed molecular dynamics simulations to understand the mechanisms of RHEED oscillations at low temperature. Classical two- and three-body Si potentials were used together with an adatom mass that is 3.84 times the Si mass to account for the Ag/Si mass ratio. Results indicate that the landing site for the adatom is very important to predict whether a particular adatom will display lateral motion or not. A fraction of the adatoms incident near a maximum of the potential energy surface display significant lateral motion. The substrate stiffness for Ag/Si results in an energy transfer process which is much slower than that in fcc/fcc systems.