Very nice!
It looks like the strips have bias edges along the sides. Bias stretches, so it is likely that the bias edge stretched as you were applying the binding. This gives a ripply edge. You can avoid this next time by heavily starching the fabric before cutting into strips. Starch will help keep bias edges from stretching out of shape. (The edges along the straight-of-grain of the strips did not stretch out of shape because straight-of-grain is more stable.)