The number of Apache busy workers on the image scalers spiked between 2:55 and 3:15 UTC, peaking at about 3:12 and overwhelming rendering.svc.eqiad.wmnet for about a minute.
The full stack trace is included in <https://bugzilla.wikimedia.org/show_bug.cgi?id=64152>, filed by Reedy yesterday. It appears File::getMimeType is returning 'unknown/unknown' and that File::getHandler is consequently not able to find a handler.