Q. What might be causing leaf browning on Cherry Laurel on some branches?
Browning of leaves on Cherry Laurel in late August probably should not be a major concern since it is occurring on some branches only. In 2016 Philadelhia area had warm weather in January followed by hard freezes in April and frosts in May. The summer had record heat and spotty rain showers. Check the bark on a branch that has browned leaves. If there is green wood under the bark it should recover next year.
Check the brown leaves for insect damage or mites. If you find any damage, contact Cooperative Extension in Philadelphia 215 471 2200. They may ask you to bring a sample in to their office.