There are several possible causes for the branch dieback you are seeing.  From your  photos it appears that your shrubs are stressed from our recent heat and a lack of water.  If a branch is damaged (by insect, animal, or mechanical means) it is possible that water movement from the roots is lessened or cutoff.  Look for damage to bark at the base of the affected branch.  Another possibility is a condition known as root rot.  A typical symptom is the death of individual branches rather than the entire shrub at one time.  Root rots generally occur in situations where soils tend to get water-logged.  I would suggest a conservative approach - remove the affected branch and be sure your shrubs are receiving adequate water.  It is not uncommon in Oakleaf Hydrangeas to have the occasional branch die.