Search engines monitor how people use their services. The data is used to assess internet usage and to target adverts or sponsored links.
A report from their gargantuan databases could list every query that was ever entered into their website from your computer. A timeline could note which link you selected and how long you remained on that site.
More pertinently, it will record which webpage caused you to click on a site and which sent you away again. If it is the same page, then it will incur a high bounce rate. This is still true, even if you followed an external link given in the content.
If someone leaves their computer idling on a single area of your website, while they go away for a reasonable period of time, then you will incur a high bounce rate. Their search 'session' will have ended, thus sending a signal back to the cookie that you are not there.
There is no industry standard for the amount of time needed for a session to end. The average is thirty minutes. It's believed that anyone could read a whole webpage in that time. Therefore if you have not moved on further into the website, then the landing page has failed to entice you further. This is a mark against it which is factored into your overall rank.
Have a look at the top of your browser. How many tabs/windows do you have open? Have they been there longer than half an hour? If so, then I have a message for you from the person who owns the site:
'Thank you very much for visiting. I love the hits that you've given me. But next time, could you please click elsewhere on my site, then exit properly before going away? Thank you in advance.'
In effect, you just threw them the ball and, while they were trying to talk to win the game, you simply walked away. The search engine's cookie, silently acting as judge and jury, marked them down as a loser.
On an entirely related note, I would be very grateful if you'd just click on my profile after reading this. Then perhaps have a look at some of my other articles? Yes, the above note was from me too.