If you pull the backing too tight before basting, that will happen. You can use Sharon Schamber's board basting method, or else -- if you're taping the backing to the floor/table to baste, make sure that the backing fabric is smooth, but not taught. I tape the top, then gently smooth with flat hands (often I use the backs of my hands to ensure I'm not pulling the fabric) to the bottom of my quilt & tape -- then I do the same thing with the right & left sides. Once that's all done, you can spray & lay your batting on top, then do the same for the quilt top.