If you ever get wavy quilts after machine quilting them, you’ve probably got some really dense quilting designs that aren’t balanced throughout the quilt. My latest quilt is almost done: I’ve densely quilted the interior, but not the frame, thus the rolls and waves you see here. Once I quilt the frame portion the entire quilt will lie flat.
I see this mostly in borders. Quilters often do stippling or some other tightly-spaced quilting design in the body of the quilt, but use a widely-spaced quilting design in the border, only to have the quilt roll on them. Try this next time and see if you get flatter results.