Well, that's annoying.
All the livelong day, YouTube videos have been autoplaying in my web browser (I’m using Google Chrome). I just opened 19 tabs at once, and my computer basically threatened to walk off the job. My other browsers aren’t affected, so this appears to be a Chrome-YouTube joint.
A fix is coming. It’s apparently a problem on YouTube’s end, and the team is aware of it. Check out this Google thread for updates.