DDoS attacks are hard to defend against because it puts a severe load on the server, and it becomes hard to distinguish between an attack and a normal 'login' attempt. They've been used multiple times in the past, by disgruntled players, by people who want to extort money from businesses by essentially holding their servers to ransom, and by others with a more political agenda.

It's precisely because it's so difficult to defend against which makes it an effective way of shutting down servers. Why these people are DDoS'ing the server is the real question, but I suspect we'll never get an answer. Probably some annoyed player who's mates left WoW to join Eorzea and he's been left all on his own as a result...