Because the Himation didn't drape properly over her arm, and instead was intersecting with the Shield. There are two arm bands on the back of the Shield, and though you can't see the second one near her elbow, the Himation crumbled badly over the Shield and that part of her arm. So instead of getting a nice drape down her left side, it was very weird.
One way to handle this would be NOT to collide with the shield, and then use the Morphing Tool to handle any pokethrus, which is what it was originally created for.