Bing has added a video background which shows changing seasons at Grand Teton National Park. The idea behind uploading the HTML5 enabled video seems to be welcoming the season of fall.
The video is 13 seconds long, and is of about 1.9 MB.
The step may be part of Bing's overall effort to go for a change in its look and feel. Just recently Bing was in news for going for a homepage makeover, wherein its prime focus seemed to be getting more user/visitor friendly.