Originally Posted by
ClairVoyantQuilter
This happens because the longarmer didn’t cut the batting to the width of the backing.
As the quilt winds on the takeup bar, the outer edges begin to flop around because of the slack and a pucker can result.
This can happen whether the quilter is using a computerized system or freehanding.
I would say this is only one of many reasons this can happen. Don't make assumptions.
For a short list: not stretching the back with clamps; if it's custom quilted, quilting from the outside in; clamping backing and batting, rather than just backing; out of square/poorly pieced backing (not your problem, from what you said); not pinned/rolled squarely on the frame; not stretched/free of wrinkles on the roller bar; not checking the backing with each roll.
As a longamer, I have to admit familiarity with most of the above
Whatever the root cause (and only the longarmer would know for sure which mistake she made), it's definitely not something I would be happy with. I'm glad she pointed it out to you and didn't try to hide it, but sorry you had to put so much extra effort into correcting the issue.