Sunspot is a relatively "short" variety that only grows to 4 feet. Autumn beauty will grow to about 6 feet. The Mexican sunflower Tithonia diversifolia (which is not really a sunflower at all, though it looks similar) grows from 6 to 10 feet tall.
I think your plants are just doing what they do, and you chose the wrong plants to grow if you have ...
According to this information paper, I should begin trimming lower branches to shape the tree after 3-4 years, but no more than 25% per year.