For example, I doubt most people watching a video on YouTube would like the video to stop playing when they scroll down. If they wanted to pause it, they'd pause it. They also probably don't want the video to transition to the next, whilst they're reading threads.
If you're in an infinite loop stream however, you do probably want it to pause or stop, without intervention, as you've scrolled away and dismissed the video.
However - you may not want that to happen whilst you're still in-thread with the video. If you're looking at comments on the video, you probably still want it to continue. But if you reach the bottom of the thread, you probably don't want to have it pause and have to find the start of the thread to continue playing it, just because you scrolled an inch too far.