What is YouTube’s auto-pause from an UX standpoint? [closed]

When I'm listening to music on YouTube after some time, it stops by itself and I was wondering why is that happening?

I considered two possibilities but maybe I'm wrong: Dark pattern to make you check on the video player and get hooked again on the website.

Server bandwidth economy mode.

A third thing would be to make you buy Youtube Red, but I was told that this also happens there.

To not display ads to AFK users which I find weird because most recent ads I heard were from Youtube l.