From Reddit

If a group gets enough reports (for nudity, spam, offensive content, etc)... even if they DO NOT contain any of the aforementioned content, Facebook's algorithm will automatically shut down the group until it is deemed worthy of being reinstated by Facebook staff after an investigation.
Now the only question is... who has, say, $1,000,000 in spare change lying around to employ a large enough amount of people to spam-report these pages in a coordinated attack? ponders intensely

EDIT: Here are some screenshots from Hillary Clinton support groups confirming my suspicions