Without more background information it's hard to tell. Big leaf hydrangea has a tendency to be sensitive to drought and we haven't had much rainfall the last few weeks. That being said, you have some discoloration in the first picture which looks like a nutrient or pH issue. Has this plant been fertilized recently? Excessive fertilization can compound drought damage. Some fertilizers can make the soil more or less acidic as well. In this case the leaves are chlorotic and look like they might have an iron deficiency. This could be caused by the addition of lime or other additives that would increase the pH. Sometimes waste concrete in the soil can cause this problem as it increases the pH.

Here is a summary of my thoughts:

• Get a soil test for this flower bed, don't add fertilizer unless it is needed
  
• Avoid watering the leaves of the plant and water from the base
• Irrigate plants during drought
• Avoid using herbicide sprays near plants, especially when windy

Please let me know if you can provide more information on whether plants have been fertilized, irrigated, or if herbicides have been sprayed nearby.