https://www.vice.com/en_us/article/597dnb/thousands-of-facebook-groups-go-secret-in-fear-of-the-great-zuccing

If you're a member of any large Facebook groups, you may have noticed that, sometime this week, a lot of them suddenly changed their status from "closed" to "secret."

This is happening because administrators of Facebook groups are worried that their groups might be invaded by bad actors who post porn or other content banned by Facebook, report that content, and, in turn, get the group deleted—a tactic known as "zuccing."