Seems to me:
1) This is not a new planting
2) If the shrub up and died suddenly, it is not likely due to issues that have been present over the life span of the shrub.
3) If the shrub up and died suddenly and herbicide has been used nearby recently, then the logical conclusion without testing is herbicide drift.
4) If you want to know with certainty what caused the die back then run tests. Anyone who says otherwise is uninformed at best.