Its mostly because of Google Universal. they are rolling out a new way of presenting results, including image/video/news etc all on the main page. if there is a Google / Youtube video relevant to the search term then it will be displayed just like a normal result. yes, the videos can be SEO'ed woot first post!
I have a couple of clients with videos on you-tube which are on the front page of a few of their keywords. Just one problem, the link goes to you-tube and not my client's site. While they are happy to have their video viewed it is not doing them any good on their site or in terms of PR or SERPs.
lets face it google know how to manipulate there own algorithm better than anybody else, if they purchase something for billions of $$ and they could increase both popularity and visability why wouldnt they do this?
if a video is relevant to the searched keyphrase and its clicked on, chances are they will watch the video on youtube and providing its well made, either click on to your website from the youtube channel or at least view your youtube profile page which should be as seo'ed and as well written as your main website. Ever since the announcement of google universal we've been making quality videos for youtube, now it provides us with some amazing traffic, not to mention the linkbacks to our profile page and youtube videos on other peoples blogs and websites.
oops forgot to mention.. universal is still being rolled out across the google network, this will take a long time, the absolute best thing you can do is get some videos made and get them into the system before your competitive key phrases catch on and do the same, it seems google is giving priority to the established videos over the newer ones in its search results, a video with 1000 views and 8 comments thats been in the system for 4 months now is ranking a lot higher than a similar video thats only got 500 views and been in the system for 2 months.