Reading between the lines, it looks like their maintenance system needed to take down several Borg clusters within a single AZ, and their BGP route reflectors all ran from the same set of logical clusters. They'd tried to set up geo-redundancy by having different BGP speakers across different AZs, but they were all parented by the same set of logical clusters, and the maintenance engine descheduled all of them together. Then the network ran okay ("designed to fail static for short periods of time") until the routes expired, after which routes got withdrawn and traffic blackholed.

They realized the issue within an hour.. unfortunately, since they took down multiple replicas of their network control plane, they lost Paxos primary and had to rebuild configuration.

(Disclaimer: I work in Azure, I just find it fascinating to look at Google's RCAs because failure provides an insight into their architecture and risk engineering.)