The poolcounter log showed a spike in "queue full" errors between 17:12 and 17:32. Corresponding with this, there was a spike in pcache.miss.revid.
So my theory is that this was due to https://gerrit.wikimedia.org/r/#/c/122847/ , which causes parser cache entries to be treated as expired even if $useOutdated = true. Perhaps PoolCounter queue overflow was at fault in T48014, but if so, that bug should probably be fixed some other way. The point of $useOutdated is to keep the site up during a news event, characterised by high edit rate and exceptionally high traffic to a single article, there's no point in retaining the feature if that goal is not met.