<blockquote>At 12:47 AM PDT on April 21st, a network change was performed as part of our normal AWS scaling activities in a single Availability Zone in the US East Region. The configuration change was to upgrade the capacity of the primary network. During the change, one of the standard steps is to shift traffic off of one of the redundant routers in the primary EBS network to allow the upgrade to happen. The traffic shift was executed incorrectly and rather than routing the traffic to the other router on the primary network, the traffic was routed onto the lower capacity redundant EBS network.</blockquote>
via <a href="http://aws.amazon.com/message/65648/">Summary of the Amazon EC2 and Amazon RDS Service Disruption</a>.