Most of the subreddits have pledged to go private — preventing outside access — for 48 hours, though some, like the 26 million-member community r/videos, have said they’ll remain private indefinitely.
Volunteer labor isn’t an infinite resource. Reddit needs to maintain that resource with an imminent IPO, but it sounds like the current CEO doesn’t think it’s necessary.
See also: this summary by Cyber Yuki (click Show More).