What is the most probable cause for the sudden appearance of a dominant allele with lethal effects in a fam?

Sudden, I can only guess that would imply not with in close enough familial range?
I suppose mutations are the way to go? Environmental changes are a likely cause for that sort of strange mutational changes. Lethal is unlikely, but maybe a new disease that the original species has not yet been in  contact with?
Selective breeding gone wrong?
Those are my guesses, hope I helped

Lethal allele generally expresses when two recessive alleles for the trait are coupled (i.e expressed as "rr"). The only way this could occur is if the parental generation were both carriers of the trait, but it was not expressed because the dominance was heterozygous (instead of rr, Rr where the lowercase r signifies that the individual is a carrier but the trait is not expressed because the dominant allele suppresses expression). When the two parents who are heterozygous for the trait (Rr x Rr) there is a one in four chance that the filial generation will yield progeny with the lethal allele resulting in death, most times fetal death.

Well the ultimate cause of any allele is mutation, so it's a difficult question to interpret. I also don't understand what you mean by "in a fam". If it's dominant and lethal then anyone that gets it will die. It won't be passed on within a family. Unless you mean it's not immediately lethal and someone with the allele could reproduce first? But then presumably the allele could be passed to anyone in the population, so I'd need to know what "in a fam" means. If you're just referring to a family as defined by last name, then it would be possible for such an allele to enter the family because someone carrying the allele married in.

So mutation is the most likely answer, but gene flow from outside whatever population you're talking about is also a possibility.

Most common sources of mutation in a gene that results in new alleles is during meiosis' chromosomal crossover, DNA replication errors, and environmental insults.
http://learn.genetics.utah.edu/archive/sloozeworm/mutationbg.html

Dominant alleles for a gene simply have more impact. In a disease state that results from a dominant allele a protein function is drastically impaired or altered to such an extent that the remaining correctly functioning allele for that gene cannot do the entire job alone. If the function is vital to life the disease state is lethal. Most alleles that fail to function properly are lethal during embryogenesis so never become part of the gene pool. However a gene not active until later in life or one that has a slow, accumulative action may become established as a lethal dominant allele. When this version of the gene is inherited, even in one copy, the disease manifests itself. Huntington's Chorea (HC) is an example that is lethal and dominant.  The gene's altered protein slowly disrupts normal nerve function allowing time for the individual to reproduce and pass on the allele to the next generation. A famous case is Woody Guthrie. He died from HC but no one knew if his son Arlo Guthrie had it until Arlo passed the age it normally appears.
http://evolution.berkeley.edu/evolibrary/article/0_0_0/medicine_05