This happened to me in the past. It is not the mounting system, but a combination of a floppy pannier and a rack that does not support the rear of the pannier. I bent some 1/4" aluminum rod from the hardware store, in a shape similar to an Axiom Journey rack, and attached it with some stainless wire and that solved the problem when using those bags. Since then I have only bought bags with stiff backing and only racks with rods that extend back enough to prevent the corner of the bag from flopping into the spokes.