Can you tell me what is causing this?
We need more information to help you, including what type of tree this leaf is from, and what the conditions in your yard are and have been.
What does the rest of the tree look like overall? Is it the whole tree involved? Is it just this one type of leaf falling?
Can you send us photos of the tree from a distance and of a branch with clear leaf arrangement?
You can attach additional photos right to this reply.