Even genetic diseases sometimes need environmental influences to develop.

For example, some people are prone to alcoholism in a hereditary way. But if they live in a society that has zero alcohol, they won't ever become alcoholic. Upon moving to a society where alcohol is sold freely and frequently, they may sink into alcoholism without having any obviously alcoholic ancestor.

I don't know really.

I think the cutting edge opinion is that it isn't known why they occur.

Not necessarily. A homozygous recessive disorder often occurs without prior family history.

Two carriers have a child together, the child has the disorder, this is how they learn they are carriers.