Here is an excerpt from the postmortem received from our upstream provider:
At 11:30 AM EST we began to monitor high CPU and latency on the core routers providing the internet access. Network engineers were on the devices and began looking for sources of CPU load. It was determined that multiple IPs were attacking a customer with TCP SYN packets. Once the attacking IPs were determined a filter was applied and traffic began to return to normal. We are investigating the type of attack and looking to implement filters and rate limiters on the edge network devices to prevent this type of attack resulting in impact to other customers. We are also working to implement better traffic analysis to prevent future attacks from creating such wide spread issues for other customers.
We apologize for any inconvenience that this issue has caused.
Network Engineer, Linode