Q. Why did my pair of oakleaf hydrangeas wither and die?
Being as your dogwood as well as the oak leaf hydrangea died, it seems likely that you may have a problem with anthracnose. Do you remember if the underside of the leaves on the plants had black, yellow or red spots or cankers on the branches? These are the symptoms of anthracnose which is fungal disease.
The other possibility is a problem hidden under the soil. Could there be a leaking water pipe or run off from a source that could be toxic or drown the roots of these plants? Salt kicked up from the road after a harsh winter could also be a culprit.
Courtesy of NYBG Plant Information