AWS have posted an update about related upcoming EC2 maintenance: https://aws.amazon.com/premiumsupport/maintenance-2015-03/
"We’ve received a Xen Security Advisory that requires us to update a portion of our Amazon EC2 fleet. Fewer than 10% of EC2 customer instances will need to be rebooted. We’ve started notifying affected customers when their reboots will take place. These updates must be completed by March 10, 2015 before the underlying issues we are addressing are made public. Following security best practices, the details behind these issues will be withheld until they are made public on March 10."