In the story Sees Behind Trees Why did the Weroance have the new contest first?

Because she's smart. If she had all of the other boys shoot the moss from the sky first, and then had Walnut see behind trees, it would have seemed like she gave him a different task because he could not perform the normal one. It would have been a consolation prize. Additionally, someone might have noticed Gray Fire was missing, and that would have totally ruined the challenge. By having this challenge first, the weroance sets it up as being more important. By having it when no one had a clue as to the answer, she sets it up as being important. Remember that the whole idea of this test of manhood is to prove that the boy can be a productive member of the tribe - meaning he can hunt to provide food. The weroance shows the tribe that Sees Behind Trees has a special talent and does not need to be able to hunt to be of value to the tribe.