There aren't many reasons for a single branch to show signs of distress. the most common is a different exposure to sun and/or heat, but it is obvious the branch is exposed to the same elements as the rest of the plant. another reason is the attack of pests. the most likely option is, though, it has broken or chipped somewhere along its base. look closely. it might also be a temporary stress caused by maybe twisting or some kind of mild damage. if there aren't any pests and it's not completely broken, it will recover.