Short answer: I don't know.and this was more about why would it kill the young? If you can breed every other morph to its self why not this one? What make these so different?
Long answer: In horses, there is a type of colour pattern gene, a form of Overo, that in the homozygous form is lethal. Why would a simple colour pattern kill the offspring? Because the genes that make that particular patterning also control the development of the lining of the intestines, and in the homozygous form the intestine lining doesn't develop the ability to absorb nutrients, meaning the foal will literally starve and die shortly after being born. Heterozygous Overos are beautiful animals. Homozygous Overos all die.
No other horse pattern gene is known to cause such inevitably fatal consequences. I know many folk who test their horses for the Overo gene (since it isn't always blatantly expressed, also there are other Overo patterns and Sabino patterns that can literally hide it) before breeding so they don't accidentally breed Overo to Overo.
Now let's look at Spiders. All Spiders "wobble" to some degree, from the ones who just tilt their head a little when rotated onto their back to total trainwreck Spiders who look like they're having a seizure 24/7. Since there seems to be no direct connection to a Spider's parent's wobble severity and it is case-by-case, I am 100% convinced based on what little I know that it is some form of dysfunction in the nervous system and directly tied to the Spider patterning. Heterozygously, you get a beautiful snake.
Homozygously, it might mean the nervous system fails completely. When I first considered adding a Spider to my collection (she is still at the breeder's right now) I spent months researching the wobble, and read in a few places that some breeders have bred Spider to Spider and found that an unusual number of eggs just fail. This leads me to consider that there may be something to this notion of it's lethality.
Also bear in mind that in some animals a "lethal" gene isn't actually going to kill them...merle in dogs is sometimes called "lethal white" which is a misnomer...a homozygous merle may be born with a visual or hearing deformity (such as "star-shaped" pupils) or impairment...they may be born deaf...they may be both blind AND deaf...or some are sadly born WITHOUT EYES AT ALL...but unless euthanized, they can live and even thrive if given a chance. Still, many (self included) consider it unethical to breed merle to merle...why risk such heinous birth defects?
Or tl;dr answer: I still don't know.
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