There might be a problem with how it was originally planted. I've seen synthetic twin girdle a tree, problems with planting depth, or just terrible soil conditions.
Look at the base and the roots, when you pull it out, for clues.
In my area, two years ago, I saw Burning Bushes dying all over the place. Some died back half way or more, others died completely. It looked like a fungal disease to me, but I never tested any samples. Being a tree guy, bushes don't get the expensive diagnosis protocol.
I have not seen this before or since.
Is this bush susceptible to Verticillium wilt?