YouTube is Testing a new automatic video feature. Not to be confused with a similar existing feature (allowing the mobile app to automatically play the next video after the end of this current), this new functionality is for videos that appear in the 'home' interface.
Videos play without sounds and instead see subtitles, giving you an idea of what's going on in the video.
While you can see a full video in this way, playback will stop as soon as you scroll up or down, resulting in the current (now visible on screen) video being played automatically.
The feature - which is only detected in the YouTube Android app so far - is called 'Play just as you browse' and can be disabled by going to Account -> Settings -> General.
You can set it to only activate when your device is connected to Wi-Fi. There is currently no information on when this functionality will be extended more widely, but perhaps it depends on the feedback it receives.