Supercooled liquids and glasses show a range of relaxation times. Nearly all glass-forming liquids show secondary relaxations, high-frequency dynamical modes of structural reconfiguration seemingly distinct from the primary alpha relaxations. We show that accounting for driving-force fluctuations and the diversity of reconfiguring shapes in the random first-order transition theory yields a new dynamical process that shares many of the features ascribed to secondary relaxations. Whereas primary relaxation takes place through activated events involving compact regions, secondary relaxation is governed by more ramified, string-like or percolation-like clusters of particles. These secondary relaxations generate a low free-energy tail on the distribution of activation barriers, which becomes more prominent with increasing temperature. The activation barrier distributions of the two processes merge near the dynamical-crossover temperature Tc, where the secondary process ultimately becomes the dominant mode of structural relaxation. These string-like reconfigurations are seen to smooth the transition at Tc between high-temperature collisional dynamics and activated events.
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