The attack has now pivoted to the The Organization for Transformative Works website and its donation site because the reaction to AO3 being down among its users is basically give it more money; both sites are now down while AO3 is back up under new configurations. Some apps are now not working but it is expected that this will be temporary.
The article says they pose a a religious and political motivated hacking group. Whatever that means. Maybe one of them saw some fanfic that made them question things.