Phantom touches would always be something to consider, but if it only ever happens on YouTube, that would seem less likely. You could try turning on the Show Touches option in Developer Options (if available), and watch for phantoms during your daily use. If it's not a developer option on your phone, then there are apps in the Play Store that can do this. Since phantom touches can be intermittent, you'd want to watch over some time, rather than expect them to show up immediately.
To access Developer Options, go to Settings>About Phone​ and tap Build Number 7 times.