Google have baked in some new changes into YouTube, which have resulted in changes to the way YouTube ranks content and then determines what closer matches what you're actually looking for.
The way it works now is that YouTube videos now get higher rankings based on how long viewers are actually watching the clip for, not how many clicks a video receives.
This is actually great, as spam videos or incorrectly titled clips won't receive higher ranks because people clicked on them. The change in YouTube's algorithm will also take into account how long users' overall visits to the site are, too. This will see videos gain a higher ranking if users not only watch the video all the way through to the end, but continue to stay on YouTube and watch other clips.