This happens when there are multiple accounts, Kinect recognition is turned on, and only one person is signed in and watching a youtube video.
As soon as someone else who has an account walks in the room, Kinect logs them in. This causes two problems:
1. The video you are watching stops. You cannot resume where it left off and have to start it from the beginning.
2. If you had to search for that video, the search results are gone. When the other account logs in, it takes you back to the Popular channel.
This is especially annoying when you are half-way through an hour long video. I would love for someone else to test this out and see if it happens to them, or if it is isolated to my Xbox.