I wonder if it is Google's visual site previews/thumbnails that you get when you click on the arrow at the side of a search result, that are doing this.
Perhaps Google fetches the crawled page from the cache and then renders that for the previews?
This was my first thought, and seems likely. They do several forms of analysis on their cache. It could even be some engineers running tests or queries that require rendering the page or at least bootstrapping the DOM.
Perhaps Google fetches the crawled page from the cache and then renders that for the previews?