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In this article, we study the variability of Si IV and C IV broad absorption lines (BALs) between two different epochs in the case of 10 broad absorption-line quasi-stellar objects (BALQSOs). By performing multicomponent analysis of Si IV and C IV broad absorption troughs, we analyse each BAL trough to find the individual and uniquely determined number of components it consists of. By measuring the radial velocity, full width at half-maximum (FWHM) and optical depth of each component, we study the kinematics, physical conditions and variability of each absorber in the line of sight. Our results suggest that Si IV and C IV BAL troughs arise from clumpy gas clouds that have similar locations, kinematic structure and physical conditions and that both ions originate in the same gas clouds. Our findings favour ionization changes over the moving-cloud scenario as the main driver of BAL variability. However, the possible contribution of other mechanisms cannot be ruled out, given our small quasar sample. Finally, we find strong evidence indicative of density stratification within the absorbing gas, suggesting that quasar outflows are far from being smooth and homogeneous.