I think it is related to the clicktoflash plugin.
When i was installing plugins and setting up my safari a few days ago, I noticed clicktoflash automatically renders youtube videos as html5. I personally want it to block all videos, unless instructed by me to load.
This was the main reason why i switched to using google chrome, I personally do want to use safari as my main browser, as i love that bounce feature at the top and bottom of pages, but all videos autoplaying, that puts me off.
i hadnt found a solution to using both clicktoflash (to get rid of flash and ads) AND not autoplaying youtube videos..