Time-resolved pump-probe measurements of the dynamics of fs-laser ablation of crystalline silicon under ultrahigh vacuum conditions have been performed using time-of-flight mass spectrometry in order to reveal mechanisms of particle emission from the irradiated surface. Strong emission of charged Sin+ clusters (n = 2-11) has been observed at low laser fluences, near the ablation threshold. With two consecutive laser pulses applied, the relative cluster yield is maximized at around 400-700 fs delay between the pulses and can reach up to 50% of the total amount of emitted particles. The particles exhibit non-thermal velocity distributions that remind a feature of the Coulomb explosion mechanism of the emission. The cluster emission essentially depends on the number of laser pulses applied with the strongest emission occurring after 30-40 pulses. The observations imply an accumulation of structural changes on the Si surface important for the emission process. Theoretical modeling based on a drift-diffusion approach has been performed to describe laser-induced excitation, charging and heating of the irradiated Si target under the experimental conditions. Calculations show that silicon surface charging is not nearly enough to cause bond breaking and thus we have to rule out the Coulomb explosion mechanism. Instead, we suggest a new emission mechanism connected with a structural phase transition on the Si surface. The transition is induced by the first (pump) pulse resulting in rebuilding of surface dimer bonds at temperatures well below the melting point. In addition, the pump-probe delays of several hundreds fs correspond to the timescale of surface structural instability due to a perturbation of the interatomic bonds and surface charging. The second (probe) pulse, applied to the modified unstable surface, acts as a trigger resulting in efficient emission of clusters. The mechanism responsible for the non-thermal velocity distributions of the emitted clusters will be also discussed.