  Broad absorption lines (BALs) in quasar spectra indicate high-velocity
outflows that may be present in all quasars and could be an important
contributor to feedback to their host galaxies. Variability studies of BALs
help illuminate the structure, evolution, and basic physical properties of the
outflows. Here we present further results from an ongoing BAL monitoring
campaign of a sample of 24 luminous quasars at redshifts 1.2 < z < 2.9. We
directly compare the variabilities in the CIV 1549 and SiIV 1400 absorption to
try to ascertain the cause(s) of the variability. We find that SiIV BALs are
more likely to vary than CIV BALs. When looking at flow speeds >-20 000 km/s,
47 per cent of quasars exhibited SiIV variability while 31 per cent exhibited
CIV variability. Furthermore, ~50 per cent of the variable SiIV regions did not
have corresponding CIV variability at the same velocities. When both CIV and
SiIV varied, those changes always occurred in the same sense (either getting
weaker or stronger). We also include our full data set so far in this paper,
which includes up to 10 epochs of data per quasar. The multi-epoch data show
that the BAL changes were not generally monotonic across the full ~5 to ~8 yr
time span of our observations, suggesting that the characteristic time-scale
for significant line variations, and (perhaps) for structural changes in the
outflows, is less than a few years. Coordinated variabilities between
absorption regions at different velocities in individual quasars seems to favor
changing ionization of the outflowing gas as the cause of the observed BAL
variability. However, variability in limited portions of broad troughs fits
naturally in a scenario where movements of individual clouds, or substructures
in the flow, across our lines-of-sight cause the absorption to vary. The actual
situation may be a complex mixture of changing ionization and cloud movements.
