Grouping entries and ordering by section
The suggestion to order by sectionId resolved my issue here. 

{% set entries = craft.entries().section('extendedEvents, festivalEvents'). orderBy({sectionId: SORT_ASC}) %}

As it was, I was working with two sections, so my quick fix was to simply reverse the order of the output of the for loop...

{% for section, eventsInSection in allEventsBySection | reverse %}