A lot of speculation is that it's revenge from banned RMTers.
People DDoS games for lots of ridiculous reasons and it's not hard to do it if you have a little spare cash.

Someone will probably shout that it's just bad servers and I don't disagree but I also 100% believe people with nothing better to do would DDoS one of the most popular games over some stupid gripe. People do really absurd stuff when it comes to popular multiplayer online games.