Over the past 24 hours there's been a problem with embedded YouTube videos seems to Autoplay automatically when using the Chrome Browser. On a webpage that hosts multiple YT videos on one page they will all start playing - very annoying.
So far there isn't a definitive reason why this started happening, but you can fix (workaround) this issue by disabling the Chrome Cast Extension. Here's the steps on how to do it.
In your Chrome Web Browser jump into the 'Preferences'.
On the left jump over to the Extensions menu and disable the Chome Cast Extension [uncheck the enable ticker].
Next you may have to clear your Browsing History. From the Chome Browser choose Clear Browsing Data.
Finally don't just close the window, make sure you Quit Google Chrome completely. If you are on Windows make sure you end any tasks with the Chrome browser. Once you relaunch the Google Chrome Browser, the YouTube videos should not autoplay.