Botnets are also hireable, so it could just be someone decided to hire a botnet for a few hours and DDoS Blizzard for whatever reason. No clue how much such a thing costs though, so I’m not sure how accessible such a thing is, but hate and anger related DDoS attacks are pretty common (though, I imagine you’d need a large scale DDoS attack to make a meaningful impact against most large companies).
Ultimately, we don’t know the reasons, and the reasons don’t matter anyway. 