CPE1704TKS tips news that Amazon has provided a post-mortem on why EC2 failed
"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. For a portion of the EBS cluster in the affected Availability Zone, this meant that they did not have a functioning primary or secondary network because traffic was purposely shifted away from the primary network and the secondary network couldn't handle the traffic level it was receiving."