I agree with Azwendyg...and you tied it so that gives it a lot of wiggle room. Does you washer have a agitator? that will give it more room to pull seams apart; you could in addition to pre-washing fabric surge the seams to lock them...Sorry that it happened. I have seen it happen a lot. I save flannel for raggy shaggy quilts or backs where it isn't cut up a lot.