You are quilting along the bias grain of the backing fabric. The bias grain is very prone to stretching.
I avoid this problem by heavily starching my backing fabric before layering a quilt. Starch stabilizes the backing and prevents puckering.
Some people use spray starch, but I prefer my own method -- especially for large pieces of fabric such as quilt backings. I mix a 1:1 solution of Sta-Flo liquid laundry starch and water, lay the fabric out on my kitchen island, "paint" the starch on with a large wall painting brush, toss the fabric in the dryer, and iron with steam.