Quaking asp surprise
While I can't tell the exact cause from this photo, I can say for certain that the cause is located at/near the base of that branch. It could be a physical/structural problem like a broken branch, it could be insect or fungus damage, or it could be something directly below that branch along the stem or with the root system. That is, there could have been some chemical (herbicide) uptake right below that branch that caused it to die.
Once the cause is determined, then a course of action can be recommended. It may simply be that that individual branch should be removed.
Wish I could be more exact. Best wishes with your tree,
Thanks for the extra pics. Unfortunately, nothing specific is jumping out at me. Wish I could give you a clear answer.