Settlements tend to be near water like rivers, lakes or oceans and the larger the body of water, the more likely there are people. A small stream is likely to join another at some point. This is why going downstream (or merely going down if there is no stream) is the safest bet if there are no other clues. Even if you're not actually on a mountain, it's more likely that streams of water will join each other than that they will split. So you should follow the trail in the direction of the stream going down as much as possible. Some situational awareness will probably come into play as well, it’s unlikely you will have absolutely no idea of the surrounding settlements. If you know you’re near an ocean where this stream might end up, or near a river that this stream might join, that would make it even easier.
As an added note, of course this is not exact science. This is about making a choice in a situation where you have two options and one of them is more likely to get you safe.