The decision was made because of Anonymous' "Operation Guantanamo", which started on May 18th to coincide with the 100th day of the beginning of prisoners' hunger strikes, and was apparently not a reaction to successful attacks but only a precaution measure against possible ones.
According to the BBC, access to social networks such as Facebook and Twitter from military computers located in the base has also been blocked.
According to a video posted by Anonymous in which they urge people to protest the continuous existence of the prison, OpGTMO was launched with the tagline "We will shut down Guantanamo", but wasn't aimed at launching cyber attacks against it.
Instead, individuals were asked to send "twitterstorms, email bombs and fax bombs" asking for the shutting down of the prison, as well as placing calls to the White House and U.S. Department of Defense and voicing the same request.
Nevertheless, Anonymous apparently considers the shutdown of Wi-Fi service as a small victory.