This might also be a feature of your browser. Firefox and chrome both implemented this a little while ago. If you pin the tab, it shouldn't stop playing video when you switch away. I forget which of the browsers, but it really helps people on bad laptops and what not.
Well, a lot of people use Youtube as a music service, so that makes sense. I think there may be a domain whitelist for non-active page video in chrome, which was brought up and discusses here a while back, but I can't recall. Or maybe it was auto-play? Maybe they share a whitelist though.
