Living in Washington I had the opportunity to visit the Warm and Natural Company where they make their batting. Warm and Natural uses a scrim to hold the cotton in place allowing it to be quilted further apart. The scrim also controls bearding of the batting. There are polyester bats that have a scrim and there are ones that don't and those can beard and migrate as well. Interesting to see what happened to your quilt. Looks like you had quilted fairly well but the batting probably had no scrim so it migrated where ever it choose.