The trigger for stuttering vary from person to person. But from what I've gathered there are three main groups. Some people stutter because they're genetically pre-disposed to develop neurological structural differences in certain speech areas in the brain. It runs in their family or they may even be the only one / first in their family. For these people, they often begin stuttering pretty much as soon as they can talk. I fall into this group and I think this is a majority of stutterers imo. The next group of people are stroke victims or people with a brain injury. Many of these people start stuttering in adulthood and may have other physical abilities comprised. People with Parkinson's can also falll in this group as some develop a stutter. And the last group is people who suffered some kind of traumatic event in their life, usually in childhood. So this can be like being attacked by a dog, verbally abusive parents, a car accident, being in a house fire or victim of arson, etc..