From the moment a caterpillar emerges, internally it begins to form the adult parts of its body. Adult Monarchs have magnetic receptors along the inner margins of their thorax, which help guide them on their south to north, north to south migration. I wonder if the magnetic receptors are here at work in the caterpillars north-south pattern of pupating.
All nine hung in a J-shape on the north-south axis as well. Since I took this photo three more have also pupated on the north-south axis. In the above photo, you can see the center caterpillar and caterpillar to the far left are in the midst of changing from a caterpillar to a chrysalis (pupating).