It is highly likely that your mother was misdiagnosed as suffering from unipolar depression when she was in fact suffering from bipolar disorder. A sudden switch to mania is a common outcome, even in cases where the patient has no previous history of mania. It is crucially important to take a comprehensive history to rule out bipolar disorder, but many general practitioners (and some psychiatrists) reflexively prescribe SSRIs whenever they see a depressive episode, even where there is clear evidence of a personal history of hypomania or a family history of mania.