Any SSRI (Selective Serotonin Re-uptake Inhibitor) may cause this sort of mood swing (emotional lability) and irritability. I don't know if Effexor is an SSRI, but you might find doing a search on "serotonin syndrome" may give you more pertinent information.
In my experience, any anti-depressant that doesn't work positively will work negatively, usually roller-coaster style, at least some of the time. But the SSRIs, particularly Prozac and Paxil, are notorious.