I think it's part of the uploading process. Is this a newly uploaded video? It's probably still just processing. When I upload HD videos, and try to view them right after, the visual usually breaks, and sometimes be completely absent for an hour or so after uploading. I don't use Youtube too much in terms of uploading, and I'm not too knowledgeable of the changes that keep being made, but I think that's what it is. The video quality options take a while to be available, because HD video files are so friggin' huge, processing the quality comes later, after the video has already been uploaded to the site. You should probably check back in a few hours and see if HD is available then.
