For future reference, that happened because as you roll up the quilted parts of the quilt onto the takeup bar, the backing gets used up/rolled up more than the top of the quilt. Not a lot, but enough to make a difference when you get to the end, especially if it's a long quilt. Think of rolling up a magazine with the spine in the center, and how the page edges don't line up---the page farthest from the center is the "shortest", when in reality they are all the same size. You just need to allow more length in your backing than you did in this case. This has happened to me once, too--one of those lessons you only need to learn ONCE, lol. Congratulations on fishing your quilt!