In detail, what causes mutations in regulatory genes?
nothing causes mutations in any specific genes. mutations actually occur as the result of random processes and can mutate any given point in the genome. mutations may be found more often in some genes or regions of the genome more often because they have a positive selective force that gives them more staying power in the gene pool over time.
Its better to think of it this way: mutations happen everywhere, but they stay in the gene pool more often if they do something useful. Genes are the active portions of the genome so mutations in genes are more likely to stay around.