my guess is that it was fertilized too late in the season and didn't
have time to harden off before fall frosts hit it. that would be one
guess. i usually stop fertilizing with slow release granules around
july 1st here in central PA. i am a nurseryman that collects and
grows many different kinds clethra as well as other unusual material.
from my experience, late fertilization. would be the culprit.