At a guess: reconnecting traffic at the billions-of-people scale has the potential for finding all sorts of weird behaviours. For all we know, it has been connected and disconnected 10x already during the outage, with each reconnect overwhelming some new, deeper level of the system each time.

Reconnect and the GLBs fall apart under load as the entire world’s cadre of recursive resolvers hit you.

Fix that. Reconnect again. This time your LBs have marked half the servers as offline because their heartbeats have been failing.

Fix that. Reconnect again. Now all the memcache data is hours old and so the site business logic fetches straight from databases, knocking them over.

Fix the databases. Reconnect again. Ad nauseam.