I do longarm quilting, and am not too experienced, I keep telling everyone that I am still in the learning stage. However, I think your long arm quilter didn't keep track of your backing at all.
Many times I crawl under my frame to check the backing to make sure there are no puckers. Different materials handle differently.
I am doing a quilt now where the backing is muslin. Talk about stretching, the material is a much looser weave than the top, so as I finish a top row I am under the frame looking for any stretching before I move to the next row if there is I try to repin where possible.
I pointed this out to the little lady that made the top, I have quilted about a dozen of her quilts. She said not to worry about any puckers, but I do and I found that if I stipple in all the real open places it pulls the backing nicely, it takes a lot longer to quilt but when you think of all the work this 77 year old lady had done on her tops over the years, you want it to look its best.