One of the major reasons for the slowdown was a tiff between Spamhaus ,a group fighting spam and Cyberbunker, a Dutch company that hosts Web sites said to be sending spam that led to one of the largest computer attacks on the Internet, causing widespread congestion and jamming crucial infrastructure around the world.
The dispute started when Spamhaus added Cyberbunker to its blacklist, which is used by email providers to weed out spam.
Cyberbunker allegedly started "bazooka" attacks which were essentially distributed denial of service attack (DDoS), that essentially bombard sites with traffic in an effort to disrupt them. As per a BBC report Spamhaus alleged that Cyberbunker, in cooperation with "criminal gangs" from Eastern Europe and Russia, was behind the attack.
The other reason was cutting of the South East Asia-Middle East-Western Europe 4 (aka SEA-ME-WE 4) undersea cable near Alexandria in Egypt, that affected users in and around Africa, Middle East and some parts of Asia. Online publication Gigaom also points out that the timing of the cut was unfortunate as other major cables including Europe India Gateway (EIG) and India-Middle East-Western Europe (IMEWE) were in "maintenance mode."
This led to network congestion. SEACOM, which looks after the cable later issued a press note saying that cause of the Internet outage was a physical cable cut some kilometres north of the coast of Egypt in the Mediterranean Sea. Mark Simpson, CEO of SEACOM said that it's difficult to know the exact cause until the cable is repaired in the coming week or two and the damaged section is recovered from the seabed and inspected, but the consortium suspected, that the most likely reason was external aggression to the cable most likely caused by a larger vessel dragging its anchor across the sea bed.