Either hackers took it down and Dropbox is lying on their status page, or Dropbox did routine maintenance and Anonymous is taking false credit.
Unfortunately, I find option #1 to be more likely.
Unfortunately, I find option #1 to be more likely.
They're saying it's an issue that "arose during routine maintenance", so that doesn't preclude the possibility they had inadvertently exposed a security vulnerability by doing that, leading to it then being exploited.