I ran into this issue today while using Firefox. I had several tabs open with two of them being YouTube videos. One of them I was watching at the moment, the other I had paused and ready to go for when I finished the one I was on. Suddenly a third mystery tab opened up and began to autoplay a video that was in my recommended list. The issue hasn't happened since, but after Googling for a bit I've come to see that a bunch of people have had the same problem in the last month or so. The only extensions I have installed are Ublock Origin and NordVPN...nothing out of this world. I've also run several malware scans with Bitdefender, Malwarebytes and Roguekiller to no avail. I'm starting to think this might be a generalized YouTube issue that has only affected a minority of users.