I've never done a quilt this way, so I can only guess.
My bet is that the rollers are taking up the quilt unevenly. What kind of fabric is the backing? I'm wondering if it is stretching as you roll and tie, then springing back when you take the quilt off the frame. Or maybe it is the top that is stretching as you roll.
Or maybe the quilt is really thick and the takeup rollers are really small. (If you roll any thick quilt on a thin rod like a broomstick handle, the inner side of the quilt will wrinkle.) This would make more sense to me if he were tying from the back of the quilt.
Since I've never done it I can't be sure, but I would think that if you roll the quilt to the middle after it is mounted and then tie from the middle out, that might help.