Google has always been interested in bounce rates, primarily to let webmasters know how readers are responding to the content on their pages. But what Google Analytics reports as your bounce rate is not entirely accurate.
Every page on your website is optimized for a number of keywords, not just one. Some of those are intentional and some are not. Just in the last 6 months or so Google has been tinkering with the way that it displays SERP snippets. In some cases the snippet is taken from your meta description, most often when the search query used matches your primary keyword for the page. But when a search query is for another keyword phrase the snippet is often quite different. Google will then use a snippet of page content where that search query phrase is used. This supposedly has resulted in increased click throughs.
If Google can measure your traffic for each keyword, which it does, and it can measure your traffic for each keyword per page, then it can measure your bounce rate for each keyword per page. It may not be reporting that to you through Google Analytics, but it’s measuring it. And if Google measures something it’s for a reason.
When a searcher types a search query into Google and finds your website, she immediately determines whether or not that’s what she was looking for. If you have one sentence on your page about the topic she was searching and that’s all she can find then she will likely back out and visit another page on the SERP. If enough users do that then you’ll eventually fall in the search engine rankings for that keyword because Google figures that a high bounce rate for that keyword for that page is a bad fit for its search results page. On the other hand, if you get a high number of searchers who visit your page after a particular search query and they stay on your site longer then that’s a good sign that your web page is a good fit for the search query. You’ll have a lower bounce rate for that key phrase for that page and you’ll rise in comparison to other pages based on their bounce rates for that key phrase.
I haven’t read anywhere that this is the case, but it does make sense and I’m pretty sure that if Google isn’t doing this already then it soon will be.