The process of going feral has been covered in Fallout games before. Ghouls aren't just created feral or "normal", and it's not the result of too much radiation. A person gets turned into a ghoul by getting irradiated at absurd levels, but not too irradiated, and then they have to mutate just right. Sometimes it creates a regular ghoul, sometimes it creates a glowing one. But they can both retain their intelligence.

The process is very traumatic though and some shift over to full feral very fast as they transform. Other ghouls are able to hold on and stay sane. But they face the constant risk of slipping and going feral. The way it's described in the games, going feral is reminiscent to the onset of dementia. Some of the game have actually indicated that high levels of radiation can actually slow the feral process and keep them healthy and sane.

One of the main reasons why they're discriminated against (other than their appearance) is that most normal humans are afraid that they never know if their neighbor might snap one day, turn feral, and then attack them and their family.