Just did the same test and I'd say it's auto play videos.
(same setup - current Chrome / current OS X)
I turned on auto play videos, killed the browser, waited for my headphones to turn off and went back into FB - scrolled down and when a video came into view, the headphones turned on.
Even though the video might have no sound, the html 5 video element still needs to allow sound to pass through. When videos auto play without side, it's not that the video doesn't have sound; it's just that the video has been muted until you click on it.
(speculation of course; I don't work for FB)
