Did you check the seams before you layered the quilt? It is possible that the seam slipped on one side and there just wasn't enough to hold the two pieces together. I make it a point to check ALL my seams as I go so that won't happen. You could carefully rip a slightly larger opening and put a piece of iron-on interfacing underneath. (It's a bit fiddely but worth the effort. Make sure that you get the seams turned under properly. After you iron the seam shut, use some tiny slip stitches to secure the seam again from the top.