"It's unclear why AT&T would block 4chan, or even if it was
a deliberate action, but assuming the Internet service provider was
blocking the site, it's not hard to understand why. In addition to
being credited with many Internet memes including the RickRoll and
LOLcats, other more dubious actions have been associated with the
Website including the recent Operation Sh**ter that manipulated
Twitter's trending topics; the YouTube porn prank; the Time 100
poll "hack" and the harassment of the Church of Scientology by the
group called Anonymous.
"As PC World's David Murphy pointed out, 4chan.org is also the
target of as many attacks as its members perpetrate. DDos attacks
brought the site down twice this month alone on June 8 and June 15.
So it's possible AT&T was blocking the site because of problems
associated with it."