I suspect it is the shrinking of the batting. Most cotton battings will shrink 3-5%. So for table runners and placemats I presoak my cotton batting in hot water and dry it before assembly. I have also used low loft polyester batting in these projects with good results and no shrinkage. I will also presoak (not wash in the washer)the fabrics in warm water in the sink and lay flat to dry to avoid fraying. With both fabric and batting preshrunk they stay flat after washings.