[republished; edited & updated]
Although we already know about the DMs, it is not solely the only reason why he left (and why he was accused of sexually harassing someone). Below are screenshots of an article from Soompi which I think will explain these matters further (containing statements from his ex who claims that he has sexually harassed):
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Produce 101: Profiles [Season 2]
SonstigesMnet's successful reality survival show in 2016, now back with a new season! Produce 101 Season 2, Mnet's new reality boy group survival show - 101 trainees' profiles, updates on the show, and more! Highest Rank: #1 in Random 170517 | 170518