It seems that a lot of users will upload video which is by default published with the default settings and thus is visible from the outside. Even if they change the settings fairly quickly, automated systems like ours will already know about the existence of that video.
There could be other reasons but this seems the most likely, especially as a video that is being uploaded can be published fairly swiftly.
[0] https://pex.com
[1] https://blog.pex.com/what-content-dominates-on-youtube-39081...