On his blog, Minecraft creator Markus "Notch" Persson explained that an attack on the server is the cause of the game's multiplayer being unavailable, adding that he isn't sure why his game was under attack.
Persson explains how this distributed denial of service (DDoS) attack works and brought down the server, comparing the way a DDoS disrupts a server to how a person paying with coins, then finding out they don't have enough, can tie up a line at a supermarket. As for why Minecraft was the target of such action, he said, "As to why minecraft.net is being targeted, I'm not sure."
According to Kotaku, a post on 4chan claims responsibility for the attack because they want Persson to provide more updates for the game. While there's no way to confirm if the post is valid, the poster claims that those behind the attacks are not active 4chan members or people from rival games. The post says, "[The attack's] purpose is to send Notch a clear message of how the future of Minecraft will turn out unless he gets to work, namely by influencing the amount of sales taking place, due to the attacks."
Picture courtesy of faultythinking.wordpress.com