Initial reports indicate that the attack was part of a genre of DDoS that infects Internet of Things devices (think webcams, DVRs, routers, etc.) all over the world with malware. Once infected, those Internet-connected devices become part of a botnet army, driving malicious traffic toward a given target. The source code for one of these types of botnets, called Mirai, was recently released to the public, leading to speculation that more Mirai-based DDoS attacks might crop up.
More background, as the attack gets parsed.