MAOIs started off due to the serendipitous discovery that iproniazid was a potent MAO inhibitor (MAOI).  Originally intended for the treatment of tuberculosis, in 1952, iproniazid's antidepressant properties were discovered when researchers noted that the depressed patients given iproniazid experienced a relief of their depression. Subsequent in vitro work led to the discovery that it inhibited MAO and eventually to the monoamine theory of depression . MAOIs became widely used as antidepressants in the early 1950s. The discovery of the 2 isoenzymes of MAO has led to the development of selective MAOIs that may have a more favorable side-effect profile. 
Ever since SSRIs were first created and marketed, clinicians have had to deal with the fallout of the occasional patient who not only didn’t respond correctly to the medication, but had intense, violent, and sometimes suicidal reactions to the drugs. In most cases, these problems were quickly resolved by stopping or altering the medication given. Suicide was uncommon in these cases, but there was no doubt that the drug did create an intense reaction in some younger patients. The presence of bipolar spectrum disorder in these individuals might indeed have been a contributing factor.