[adamanteus]

In my experience thiamin deficiency causes rather more extreme symptoms, such as convulsing and a more 'cork-screw' type of locomotion. In advanced cases I have seen Garters coil themselves so tightly that they actually prolapse. If the snake were new to you, I would suspect early deficiency, but in view of the time you've had them and your familiarity with correct diet, I would guess this is a simple birth defect brought about through selective in-breeding. As Roy suggested, a recessive gene.