On rare occasions YouTube will play exceptionally long videos as ads. When YouTube Red came out I got multiple entire hours long shows as ads (as a “free preview!”) I’m pretty sure Ive gotten one of the movies they put up for free viewing as an ad before.

Obviously you can skip after 5 seconds or whatever but they hope to catch someone playing stuff in the background. Probably to increase their crappy view count for those features to sell actual ads later.