It looks like your quilt was tied. This allows for a lot more wiggle room in the washer/dryer and will allow the seams of a loosly woven fabric to fray apart.
So, When piecing with flannel I take 1/2 inch seams instead of 1/4 inch seams especially if you ar going to tie it rather than quilt it.
Quilting will help prevent this, even if its just in the ditch quilting.