Originally Posted by
QuiltE
If your quilt was truly laying flat before binding .... what you have described could be a sign that the binding was pulled too tight as you stitched it in place.
IOW that the binding is now shorter than the quilt it is attached to
...... and now the two are fighting it out and causing the waves!
I think that is backwards. If you tug a little on the binding, it will never be wavy. If you pull too much, the center bags a little. I did that once.
When you add your binding, lay it on the edge with a little tug, hold that tug in place as you sew up to your fingers. It needs to be taut as you sew.