WordPress.com was subject to a massive DDoS attack, the largest that it has ever experienced. The size of the attack is multiple Gigabits per second and tens of millions of packets per second. DDoS attacks are usually mounted from botnets a number of computers that have been infected by malware that, given certain instructions or at a preset time, flood the target site with excessive amounts of traffic. The founder of WordPress, Matt Mullenweg, has informed them that: “This is the largest and most sustained attack we’ve seen in our 6 year history””]
Source: http://www.h-online.com/security/news/item/WordPress-hit-by-massive-DDoS-attack-1202115.html