The primary cause was deemed to be failed stock investments. One source explained that he had invested large amounts of money from friends and sunbaes in stocks, but lost it in the recent stock market crash(es). While there are likely other factors related to his suicide, this is being considered the primary one. Apparently he had been overwhelmed with the stress, although he been attempting to find ways to rectify the matter as recently as the day before his suicide.
Lee Seo-hyun (born Lee Jong-hyun) was 30 years old. M.Street was a five-member vocal group that debuted in 2004 and put out singles “Noel’s Gift,” “Start,” and “Tension.” Its members had been in recent preparations to release a new album, and Lee was said to feel the brunt of the stress of their comeback as the group’s leader.
The other entertainment site speculates his coming out of the closet recently may have had something to do with it, too.