It's the nature of the beast. Even cut exactly on grain, the threads of some fabrics will separate (you just have a longer thread). Using a bit of spray starch may help hold it together.

Why do you pre-wash? If it is one of the well known quilting fabric manufactures I cannot see why! I never do and after 7 years of a huge amount of quilts, like 100s I have never had any problems, yet! :)

If I prewash fabric I have a hand wash cycle on my washer. No fabric threads at all. Moda is the worse about unraveling for me. If your fabric is homespun it's suppose to unravel, it's great for rag quilts.

I serge the edges of my fabric before I prewash..that solves that problem for me. I find when I handle pieces more, I have more trouble with fraying. I try to only handle them/press them as needed. The age old wash/don't wash will be with us forever. (I wash because I had a quilt run ..yes it was GOOD fabric and I paid good money for it. The quilt was ruined. That fabric was washed ..and washed and WASHED. It never stopped running...so it ran into the TRASH!)