I had a FQ that 'bowed' after washing. When I laid it down on the grid of my cutting board one corner was about 4" higher than the one 18" away.
As said above the fabric, is sometimes distorted when put on the bolt. It can be full of sizing and seems absolutely perfect while sewing with it. The trouble shows up after it's been washed and relaxes back to it's originally woven state.
Some unexplained ripples are because of this. If you're lucky you can quilt it out. If not it might just look a bit homemade no matter what you try to do.
If you've made a garment, the side seams may hang crooked and there is nothing you can do to correct it other than cut it down alot for a child or add to your quilt stash.