My smoke tree
It is not possible to give you a definitive answer on the cause of this problem. Diagnosis of tree disorders from a photo is not always possible. You are welcome to bring a branch sample into the office for a more thorough look at the plant tissue in question.
If I had to guess, I would say herbicide damage was the most probable cause of this distorted growth. Herbicide drift either through sprays or applied to surrounding turf could very well be the culprit. If the contamination is minor, prune out affected branches after the tree blooms. The tree may need to be replaced if it is heavily damaged by herbicide drift.
Feel free to contact our office if you have other questions.
Let me know if I can help you further!
Jefferson County Cooperative Extension Service
810 Barret Ave
Louisville KY 40204