Why do youtube vids im watching sometimes go into multitask?

Recently , when I'm watching youtube videos, the video would go into multitask mode, meaning it turns into a small rectangle in the corner. It happens even when I'm not touching the phone, which is a Galaxy S9+. I'm watching the videos on the Youtube app and not on a browser. When the video goes to multitask, the background goes to the last app, or when no apps are opened, it goes to the home screen. This problem is pretty annoying so it would be great if I could fix it.

Welcome to Android Central! That's Picture-in-Picture (PIP) mode. It happens when you tap Back while watching a YouTube video. If you're not tapping Back yourself, it's possible the phone is experiencing phantom touches. Do you have a screen protector on?