why are dogwoods dropping leaves?
There could be multiple reasons for this, but most of them are stress-related, including drought, poor placement or disease or pest problems.
Are they showing fall coloration before dropping?
Dogwoods can have many problems in our landscapes, with leaf-spot diseases, powdery mildew, and borers being prevalent. The older the trees are, the more problems they tend to have.
As far as the drooping leaves on one and not the other, even if they had the same problems, there are genetic variances among individuals that can result in that.
If the leaves colored before they fell, take a look here: https://extension.umd.edu/hgic/early-fall-coloration
Powdery Mildew is covered here: https://extension.umd.edu/hgic/powdery-mildew-trees-and-shrubs
and one of the most common dogwood leaf spot diseases, which had a banner year because of our wet weather: https://extension.umd.edu/hgic/spot-anthracnose-dogwood-or-elsino%C3%AB-leaf-spot
These can co-exist with powdery mildew.