Yeah, the same thing is happening to me.
If I am playing a YouTube video and I have a second YouTube tab open, audio from a video (Yes, even completely random videos I've never even watched before.) will start playing in the background.
It only stops when you click on the second YouTube tab and then go back to the original tab you were watching.
However, I've also noticed that this isn't even triggered just by being on the YouTube tab at all, if you leave a YouTube tab open and go to a completely different website, the tab will start playing sounds from random videos as well.
I have a feeling this has something to do with the latest update.