Backing should not be stretched; it should just be smoothed out. Spray basting will keep things together better than pins because there is continuous contact between the layers. Did you have to "manipulate" the sandwich as you quilted? I found out the hard way that over-handling, pushing, pulling, stretching, etc. all can contribute to distortions in the quilting. One technique that can help is lifting up the quilt sandwich as you allow it to feed to the machine.