This may not be the same thing your seeing but I caught this smallie last year and emailed the pict to Jim Negus with TWRA. He said this is a common spinal deformity called Lordosis similiar to Scoliosis. caused by congenital birth defect. He also said they see this often in both smallmouth and sauger. I measured this fish at 18". It may have went 19+" if the spine had been straight.
Some more info from TWRA website