Sounds like browser prefetching, see WordPress core tickets #12603, #14382, and #20192.
Basically, sometimes some browsers see what the next post is, and decide to go ahead and load it while you are reading this one. So when you go to the next post it loads immediately, instead of you having to wait. The downside, as you found in this case, is that sometimes this has unexpected side affects if the user doesn’t end up viewing the other post.