The first thing that crossed my mind is how big is your seam allowance? If it's much less than a scant 1/4", then that can happen. I've had it happen in a couple of places on a quilt that was washed for the first time and had to repair it by hand.
I also wonder how big your stitches are - if they're too big or too loose (check your tension), then that, along with a skinny seam allowance can cause the seams to fray and pull away.
Ha! Great minds think alike...we were posting at the same time!
