The video itself looks like slop with a bad title...However, there is an article that is discussing a theory researchers came up with: Something Weird Happened to Men 7,000 Years Ago, And We Finally Know Why this article provides a link to this paper: Cultural hitchhiking and competition between patrilineal kin groups explain the post-Neolithic Y-chromosome bottleneck
We'll never know the real cause, as there is a lot of uncertainty surrounding these types of finds in human DNA; the definitive evidence has long been lost to the ravages of time. At best there is room for speculation and no concrete answer to be found.