On Saturday 10 June, ops noticed that disk space usage on the parser cache servers was rapidly increasing. An alert was triggered.
Removing binlogs and running the purge script earlier than normal (it is a weekly cron job) was the temporary solution.
The current theory on why this is happening is that it is due to https://gerrit.wikimedia.org/r/#/c/354504/ . Before that change, on page view, if the parser cache entry was found to be expired, a parse would be done and the result would be saved back to the parser cache with the same key as the old entry, so there was no significant change in space usage. But after that change, an expired entry with the old key would not be deleted or replaced, rather it would be left in place, and a parser cache entry with a new key would be written. So MW is trying to slowly duplicate the parser cache.
On July 25, a similar incident happened, this time due to Wikidata demand, apparently: T167784#3473685