What causes perimenopausal mood disorders?

Not exactly known. But probably changes in estrogen and Progesterone which have effects on the functioning of serotonin and norepinephrine. This is probably how replacing estrogen and using antidepressants cause an improvement in mood, but like all mood disorders, the exact process in which mood is produced and controlled is not really known.
Mood disorders. Most cases are unknown, but likely due to hormonal changes around that time.