What's happening is that the top layer is stretching as you sew. It doesn't become apparent until you have to cross another line of quilting; that's when the excess fabric that has accumulated in front of the presser foot turns into a tuck. Spray basting and glue basting help prevent this because, unlike pins, these provide the layers with continuous adhesion. Pins, in contrast, are connecting the three layers only at the pinned locations.
One thing that helps a lot to prevent these tucks is starch. Since your quilt is already basted, what I would recommend is laying it out on a large flat sheet and adding layers of spray starch. You can use a fan to speed drying between layers. Starch both the backing and the top. Starch stabilizes the fabric so it is much less inclined to stretch as you sew.
Next time you may want to spray starch the top before layering. It's also a good idea to heavily starch the backing before layering. I starch yardage with a 1:1 solution of Sta-Flo liquid laundry starch and water, "painting" this on with a large wall painting brush. I leave it for a few minutes to allow the fibers to absorb as much starch as they can, then toss in the dryer and iron with steam. This provides a much heavier stabilization than spray starch can offer. However, layers of spray starch can help a *lot* in a situation like this, where the quilt is already sandwiched.