We present absorption variability results for 134 bona fide mgii broad absorption line (BAL) quasars at 0.46~$lesssim z lesssim$~2.3 covering days to $sim$ 10 yr in the rest frame. We use multiple-epoch spectra from the Sloan Digital Sky Survey, which has delivered the largest such BAL-variability sample ever studied. mgii-BAL identifications and related measurements are compiled and presented in a catalog. We find a remarkable time-dependent asymmetry in EW variation from the sample, such that weakening troughs outnumber strengthening troughs, the first report of such a phenomenon in BAL variability. Our investigations of the sample further reveal that (i) the frequency of BAL variability is significantly lower (typically by a factor of 2) than that from high-ionization BALQSO samples; (ii) mgii BAL absorbers tend to have relatively high optical depths and small covering factors along our line of sight; (iii) there is no significant EW-variability correlation between mgii troughs at different velocities in the same quasar; and (iv) the EW-variability correlation between mgii and aliii BALs is significantly stronger than that between mgii and civ BALs at the same velocities. These observational results can be explained by a combined transverse-motion/ionization-change scenario, where transverse motions likely dominate the strengthening BALs while ionization changes and/or other mechanisms dominate the weakening BALs.
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