The ability to control the plastic deformation of amorphous metals is based on the capacity to influence the percolation of the shear transformation zones (STZs). Despite the recent research progress, the mechanism of STZ self-assembly has so far remained elusive. Here, we identify the structural perturbation generated by an STZ in the surrounding material and show how such a perturbation triggers the activation of the neighboring STZ. The mechanism is based on the autocatalytic generation of successive strong strain and rotation fields, leading to STZ percolation and, ultimately, to the formation of a shear band.