So the same corn snake pair produced some kinks in two different years (one year high percentage, another year lower) and other clutches incubated under the same conditions where fine? Then you may well have something genetic. If it where just one year I would think it could be some fluke like the pre lay temps in the female's cage. Hard to explain how the genetics might work. Maybe a recessive gene that they both happened to have and just very unlucky the first year to get a higher than 25% rate. Or maybe there could be some incompatibility between the two, something like an rh factor difference in humans.