Lack of oxygen to the diaphragm doesn't cause stitches because blood flow to the diaphragm is not shut off by running. Gas stretching the colon doesn't cause stitches because the pain does not disappear when you relieve yourself. A swollen liver capsule is not a cause because the liver is not swollen when you have a stitch. Thick fibrous bands called ligaments extend downward from your diaphragm to hold your liver in place. When you run, your liver drops at the exact time that your diaphragm goes up, stretching the ligaments and causing pain.
Humans have a fixed pattern of breathing when they run.
They have a 2 to 1 breathing ratio, they breathe once for each two strides. So, they breathe out when one foot, usually the right, strikes the ground. When you breathe out, your diaphragm goes up, and at the same time, the force of your footstrike causes your liver to go down. This stretches the ligaments that attach the liver to your diaphragm, causing pain. When you get a stitch, stop running and press your hand deep into your liver to raise it against your diaphragm.