Why do my legs itch when I run sometimes?

Increased blood flow. Running will increase blood flow and muscle activity and can be the cause of skin itching.
Blood flow. Itchy legs can be caused by histamine release due to increased arterial pressure from your muscles squeezing the blood vessels. It can also be caused by varicose veins and increase perspiration. The common link is altered blood flow. You may consider getting an ultrasound of you lower legs.