I agree with Iceblossom, that washing could help. Fabric is rolled onto bolts when wet, which makes the fibers distort. Also, the selvedge is much heavier on wideback. Is the selvedge edge measurement shorter than the center measurement? I have had luck with removing the selvedge and zigzagging the edge to prevent fraying.
Are you using this backing on a longarm? My friend and mentor will rip her wideback and attach top and bottom to her rails, then lightly spritz the whole thing with water as she rolls it back and forth. She might have to do this a few times but so far, she's always been able to get it back into reasonable shape.