When two hikers are traveling in opposite directions on a narrow trail, the hiker going uphill has the right of way. This rule exists to minimize impact on the environment and to make hiking safer for everyone involved.
Uphill hikers have a more difficult time because they are constantly fighting gravity. Going downhill is significantly easier, so downhill hikers can afford to be more flexible when it comes to yielding the trail.
This rule also helps to prevent accidents. When two hikers are coming towards each other on a narrow trail, the uphill hiker is more likely to see the other person first and can stop before there is a collision. The downhill hiker might not see the other person until it's too late to avoid an accident.
So, next time you're out on a hike, remember to yield to uphill hikers! It's good manners and it could save someone from getting hurt.