How do they do this? The search on YouTube is largely based on tags. So if you add the same tags to your video as you do a viral video, you could show up when someone searches for that video. By gaming the Google search algorithm, you can produce a lot of traffic. This video's tags are as follows:
So if you are searching for Bieber's birthday and come across this video, you might clip on it out of curiosity or perhaps you want to see more of what she is displaying. Most Bieber fans are probably going to ignore it. This particular video has over 25,000 hits so someone is watching it. The video itself is nonsense and is not worth watching. Apparently, these woman make a lot of these and don't care if you watch the whole video, they just want the clicks.
Why do they do this? Clicks can equal money when it comes to YouTube. If you get a lot of clicks, in this case thousands, you can get money from Google. A reply video is usually relevant to the original video, unlike these are just someone using their cleavage to help pay the rent.