I did my Grinch quilt stitch-in-the-ditch, and this happened in one spot where my seam was a little narrow. It was my first machined quilt, and since I didn't want to risk it again in the next wash, happening in a new spot, I went around all my stitches again but with a zigzag stitch (I probably could have done an echo stitch, but didn't think about that at the time.)
Don't know why yours happened, but mine has held up great since I went back over it with the zigzag stitch! I'm not sure I'm going to do stitch-in-the-ditch on another quilt that I know will get a lot of use. It might be fine, but I like to only make the same mistake twice, and I've made that one now.