Trauma doesn’t have to be severe from an adult perspective for it to cause dissociation in a child.
To a very young child, getting separated from Mom in a grocery store can be extremely traumatic. With a few months or years added to his age, it becomes a matter of him going to the manager’s desk and asking for help. When he’s a little older he might approach a clerk and ask for his mom to be paged. As the brain develops more reasoning ability, the child learns how to deal with this level of trauma with more logic and effectiveness.
A child who has learned habitual dissociation might disconnect when he loses mom in the store and not remember anything between the time he got lost and when mom appeared again. However, dissociation is much less likely to occur in a child who has felt safe throughout his young life.
What happens when the dissociative child grows up?
If someone learned to dissociate at a young age due to repeated trauma, that response establishes a pattern and tends to continue. Since this behavior began at such a young age, it becomes an ordinary part of life to him. Even in adulthood, a dissociative person generally doesn’t notice when it occurs. He may wonder about gaps in his memory but won’t have an answer for them. Those gaps could even seem normal to him; he could assume that everyone has them.
Dissociation does not require a conscious decision or choice once the pattern exists. One cannot simply demand that an adult stop dissociating. A person will always default to a habitual activity when challenged or threatened. It’s just human nature.
If someone has not learned to dissociate at a young age, will she dissociate for any reason as an adult? Maybe. It’s a lot less likely, but if the trauma is severe enough, dissociation could still occur as a voluntary or an involuntary reaction.
For example, a woman who suffers a violent rape may forget the face of her attacker even if she saw it clearly. She will remember the time before the actual event and the time following it but will “forget” at least part of the incident itself. Seeing the perpetrator’s photo may stimulate the memory to come forward into her conscious memory. Smelling something similar to the attacker’s odor or hearing a voice similar to his may bring the memory forward. It was not forgotten, just separated. It needed a stimulus to integrate it into her conscious memory. 
The horrors of war can lead someone to make the choice to dissociate from the terror. His life is in jeopardy and he can do nothing to stop it. Torture victims have spoken of feeling disembodied when they revisit the memory; this also is a manifestation of dissociation.
Is dissociation a choice or a reflex? Depending on the person and the circumstances, it can be either.
part 3 of a series on dissociation
part 1| Do You Hear What I Hear?
part 2| Go To Your Happy Place
all illustrations courtesy of freedigitalphotos.net