I think the creation of a new, split off consciousness is a coping skill the brain can acquire in early childhood when the brain is malleable. If this ability to divide the consciousness is not learned before the age of about 5 or so, it can no longer be learned because the brain / mind is not flexible enough.

However, once the brain has learned how to use a partitioning of the consciousness like this, it retains that ability. So if a person is already multiple (has DID / multiple personalities) and they suffer a severe trauma at age 13 or 23 or 33, a new division and new consciousness may come into being.

A new consciousness can even come into being in the absence of a specific trauma, in a person who is already multiple. But none of this happens unless it began with severe and/or chronic trauma in very early years. This is just my personal understanding, but I hope it helps.