TLDR: the team forgot to update the resource quota requirements for a critical component of Google’s authentication system while transitioning between quota systems.
"As part of an ongoing migration of the User ID Service to a new quota system, a change was made in October to register the User ID Service with the new quota system, but parts of the previous quota system were left in place which incorrectly reported the usage for the User ID Service as 0."
That's not the TL;DR is it? It seems that the quota system detected current usage as "0", and thus adjusted the quota downwards to 0 until the Paxos leader couldn't write, which caused all of the data to become stale, which caused downstream systems to fail because they reject outdated data.
