The photos you sent appear to be leaf scorch;  a common physiological problem of hydrangeas. Typical symptoms include yellowing leaves, or irregular dry, brown blotches. Leaf tips and margins may turn brown, as well. Severely damaged leaves may drop from the plant. Leaf scorch results when the leaves lose water faster than it can be supplied by the roots, so wilting may occur before scorch is noticed. Common causes include inadequate watering, exposure to strong light (sunlight or reflected light), high temperatures, or dry, windy conditions. Scorch can also occur as a result of damaged roots or stems, such as from an injury, compacted soils, or overwatering or overfertilization. Leaf scorch is most common on leaves farthest from the roots and those most exposed to harsh conditions--often the top of the plant and the sunniest side are most affected. Occasional minor damage is primarily an aesthetic concern, but severe or recurring damage may indicate an underlying problem with the plant's health or planting location.  

Also, check the moisture around the roots. These plants like lots of uniform soil moisture. If it was a potted plant that got transferred to the garden often the soil mix is very high in peat and tends to dry out quickly and hard to re-moisten. Or there may have been a very root bound root situation. Dig the plants up and check the original soil ball. If dry and root bound; loosen the roots lightly with your hand, soak the root ball in water till moist and then plant. Keep the soil uniformly moist for best growth. Light shade is helpful.  

Other remedial options may include:

• Improve soil drainage
• Apply an organic mulch over the root zone to help maintain soil moisture.
• Plant hydrangeas in appropriate locations. Most prefer dappled shade, especially during the hottest part of the day. Plants near reflective surfaces such as walls and fences may be more susceptible to leaf scorch.  Or be prepared to water them more frequently.
  
• When practical, consider moving those plants that routinely suffer from leaf scorch.
• Light summer pruning will help reduce the water needs of the plant, but may also reduce flowering. This will not resolve the underlying cause of the damage, but may help maintain plant health in the short term, until it has a more extensive root system.
• Provide adequate water for plants, particularly those planted in sunny or windy locations.

Additional information about hydrangeas can be found at the following sites:
http://ohioline.osu.edu/hyg-fact/1000/1063.html
http://web.extension.illinois.edu/dmp/palette/080727.html