Washing in a machine that is small or has an agitator causes more strain on the quilt also. I wash quilts in a large front loader washing machine,shorter cycle and higher spin so it gets dryer.

Check your thread tensions - occasionally one of my machines "adjusts itself" - so the stitching is not inerlocking at all - and will pull out very easily.

As others have said - check both sides of your seam - once in a while the bottom side of the seam wanders -

I know many do not agree with my way, but it has worked for me. It's not always the length of your stitch (however I agree with shorter stitches) or if your bottom fabric slipped to be narrower than the top fabric. To me, I find that BACKSTITCHING at the beginning and ending of EVERY seam helps deter that 'separating of seam' issue. I don't believe that just because another seam crosses that first seam, that it will secure it. That's why I don't like to sew strips and then SUBCUT my squares for a block. When I made a queen size RAIL FENCE quilt; by the time I was to sew all the blocks together, the seams were starting to 'separate' and I went back and secured every one of them with a backstitch of sewing. Took longer to do that than if I would have cut the individual strips and sewn them (with backstitching) for the individual rail fence blocks. Even when I sew squares together for a 4-patch or a 9-patch, I backstitch at the beginning and ending of each little square. It doesn't make it any bulkier than some people thnk it would. I don't sew for hire, so I'd rather take that extra time to do it this way. That's my story and I'm sticking to it. {chuckle}

Don't know how you quilted it but if you pressed the seams open and then Stitch in the Ditch that would cause separation. I use to sew with a 2.5 stitch length until I saw some of Leah Day's videos. She used a very short length, 1.7 or 1.8 I think. Since I started using this length I have not had any problems

It could possibly be the stitch length that is giving you problems. Set your length to 2.0 and see how that works for you.