You may just have reached the stage of there being enough Toggenburg to switch over to Togg colour.
Kind of, but more accurately in diploid organisms such as many animals are, it is often the case that one gene is dominant. But there is of course only two inherited versions of each gene (allele) within our genome. The genes that control goat coat colours are reportedly relatively simple and have several combinations. What is known as the famous "Agouti locus" present in mice and other animals is implicated. But, the white gene saanen where present is, and I quote, "completely dominant".
You could in theory have a mutant goat with a genotype of 99.9% togg but with the newly mutated white allele and it will still appear white. You could in theory also have a goat with 99.9% sannen genes with only two copies of a mutant togg like allele (although such new mutations are highly unlikely).
In this case what is likely to have happened is that in a cross bred nanny, white being dominant, always trumped the togg colour alleles until by random chance these kids ended up with two copies of togg colour alleles required.
There are a few papers out there on goat coat colour, including:
Asdell, S. A., and AD Buchanan Smith. "Inheritance of color, beard, tassels and horns in the goat." Journal of heredity 19.9 (1928): 425-430.
Hards, E. R. "The inheritance of coat characteristics in the goats of the British Isles." Brit. Goat Soc. Yearb. (1950): 67-69.
Adalsteinsson, Stefán, D. P. Sponenberg, S. Alexieva, and A. J. F. Russel. "Inheritance of goat coat colors." Journal of Heredity 85, no. 4 (1994): 267-272.