They say they're doing it in honor of Aaron Swartz[0]. (It's the eve of his death).
See: http://techcrunch.com/2014/01/10/dropbox-offline-hacker-grou...
and also see dropbox's blog post, explicitly stating that it "was caused during routine internal maintenance, and was not caused by external factors"
[0]: No.
[0]: http://daringfireball.net/projects/markdown/syntax#link