First thought. Was the binding cut on a true bias? Did you press binding before attaching?
I cut width of fabric unless the design in the binding fabric is directional. If I am following a pattern on the strip then I might cut the length wize. If I am following a distinct pattern (or fussy cutting) the binding strip then I will follow the design and not the grain of the fabric. This can contribute an attractive touch to the edge of the quilt.