It can be either a little too much fabric in the border or too much fabric in the binding. I like to attach my binding with my walking foot so all the layers move at the same rate. I use a slightly larger stitch length because I am sewing through thicker layers. I like to hand stitch the binding to the back or use a stitch that grabs the binding but doesn't add another row of solid stitching to the quilt or runner edge like a hem stitch or similar.
Your quilt is pretty and that little bit of wave will not be noticeable unless it is laying out flat like on your floor. I find when doing quilting designs on runner, that a linear design on the borders works best. A design with lots of curves and curls seems to stretch the fabric on the bias causing some ripples on the edge.