Right now they seem to be in random order, or at least the order they were added, which might as well be random. Look at this extremely long list from the butt_pose wiki entry for example:
The following tags are aliased to this tag: t'n'a_pose, boob_and_ass_(pose), boobs_and_ass_pose, boobs_and_ass_(pose), boobs_n_ass_pose, boobs-and-ass_pose, tit_and_butt_(pose), boobs-and-ass_(pose), tits_n_butt_pose, boob-and-ass_(pose), boobs_'n'_ass_pose, boobs_'n'_ass_(pose), tit-and-butt_(pose), boobs_'n'_butt_pose, tits_and_ass_(pose), tits-and-butt_pose, tit-and-ass_(pose), boobs-and-butt_(pose), tits_and_butt_(pose), ass_pose, boobs'n'butt_pose, tits_and_butt_pose, boobs_n_ass_(pose), tit_and_butt_pose, tit-and-butt_pose, t'n'a_(pose), tits_'n'_butt_(pose), tits-and-butt, tits_'n'_butt_pose, tits'n'butt_(pose), tna_(pose), boob-and-ass_pose, boobs'n'ass_pose, boobs_'n'_butt_(pose), tits_n_ass_pose, tit-and-ass_pose, tits-and-ass_(pose), boobs'n'butt_(pose), tits'n'ass_pose, tits'n'butt_pose, tits_and_ass_pose, tits_'n'_ass_(pose), tits_'n'_ass_pose, boob_and_ass_pose, tit_and_ass_(pose), boobs_n_butt_pose, tits_n_ass_(pose), tna_pose, boobs-and-ass, boobs-and-butt, tits-and-ass, boobs'n'ass_(pose), tits'n'ass_(pose), tits-and-butt_(pose), tits_n_butt_(pose), tit_and_ass_pose, tits-and-ass_pose, boobs_n_butt_(pose) (learn more).
If I wanted to find a specific tag in that list, I'd have to pull up the browser's search popup. They should be alphabetized for the same reason tag lists in the sidebar are alphabetized.
