This poem is about a turning point, and the choice of season reflects this. The speaker finds himself in a "yellow wood," where we can assume that the leaves on the trees have begun to turn, but have not yet deepened to brown, nor fallen entirely from the trees. The autumnal season is beginning, but it is in its infancy, with the path still "grassy." Although a big change is coming, it has not yet gone beyond the point of no return.
We know that some "leaves," not yet "trodden back" by any passer by during this season, have fallen onto both the paths that confront the speaker. This signifies that, although the decision has not yet been made and the season is not yet fully underway, a commitment has been made. At this juncture, it is too late to turn back, just as it is too late to reattach the leaves to the trees: a path must be chosen.