Last year YouTube started rolling out custom thumbnails to more of its users. Notice a theme?
It’s important to note that most of these thumbnails are chosen by users, not the movie studios, but surely studios can’t be surprised when this happens. Given how out of place and unrepresentative most of these shots are—I’m looking at you, Star Trek Into Darkness—it seems likely that some of these shots are included in the trailers specifically to attract a few extra clicks.