I think this happens when there is not enough side tensioning on the quilt. I have a frame and am still struggling with getting the side tensioning right so it does not interfere with my quilting. The quilts I have done without side tensioning all have the problem you describe (except it is not so noticeable because the borders aren't directional). Ideally I need to measure the quilt's width throughout the process so that the middle of the quilt is stretched as much as the bottom/top (which are attached to the rollers).