Quilting lines along the diagonal can pop because fabric stretches along the bias whereas the stitching does not. When someone sits on a quilt, for example, the fabric is subject to the kind of stress that will stretch it. The solution is probably to have quilting lines closer together to distribute stress over a larger area and to limit how much the fabric can stretch along the bias between quilting lines.
One of the difficulties of quilting lines on the bias is that the fabric (especially the top fabric) has a tendency to stretch as you sew. Using a walking foot helps a lot, but does not entirely eliminate the problem. What also helps a lot is heavily starching the top and backing fabric before layering. Starch stabilizes fabric so it is less easily stretched along the bias.