There are small insects present. Various insects can cause yellowing foliage. They could be sucking insects called scale, which can cover the branches in severe infestations. They can be difficult to control once they reach adult size. Natural predators can control scale.
Supreme oil can be effective if applied at the right time.
My redwoods look stressed and the ends of the branches are dying. The strong spring winds caused drying out of the tender terminal growth and the results are now visible. If the trees are planting in the lawn supplement the lawn irrigation so that the redwoods get a deep watering about once a month to a depth of 2-3 feet throughout the entire root zone. Generally, they are ready for harvest from early August to late September. The best harvest method is to spread a tarp under the tree and hit the branches with a pole.