Outflows of material ejected from the proximity of supermassive black holes are the progenitors of the large-scale feedback that regulates the evolution of massive galaxies, but little is still known about their nature and origin. We present results from composite outflow absorption spectra built from 60,000 broad absorption line (BAL) systems in the BOSS/SDSS DR12 quasars. First, all the absorption lines in our composite spectra show a signature known as line locking, driven by the CIV atomic doublet, which confirms that radiation pressure is a prevalent mechanism at accelerating the outflows. Second, correlations of the absorption equivalent width of certain species with the detection of metastable excited state transitions, electron density, and outflow velocity suggest the presence of collisional processes and radiation-pressure waves in the outflow. We can explain all these results considering a novel stratified multiphase outflow structure, where the dense cores of the waves can cool and give rise to the detected low-ionization species.