Having had a read of Fallout Wiki, it doesn't explain what caused some people to transform into ghouls, where others simply died from radiation poisoning. There is talk about a certain "genetic factor" that somehow activates in periods of high radiation that causes the human body to transform - a protective mechanism to allow the person to survive in an irradiated world.

You may ask how such a genetic factor come about, and reach millions of people by 2077, from a divergent timeline in the 1950s. If these were inherited genes caused by a mutation in the 50s, then only the descendents of the mutant could become ghouls. This wouldn't be possible, and so worldwidely spread if this was the case.

I've thought of another cause.

Consider the case of the Human Immunodeficiency Virus (HIV), the world's most-feared sixually transmitted disease. HIV is a retrovirus, meaning it can insert itself into a person's DNA and reactivate at some point in the future to cause the symptoms of AIDS.

What if there was a retrovirus, that spread itself harmlessly between people, causing no apparent symptoms? People wouldn't be looking for it, so it wouldn't be detected, but the virus only activates when the carrier is exposed to high levels of radiation, causing the body to change...