When it starts being involuntary and harmful, or perhaps in response to emotional trauma?

All personality disorders contain normal traits; you’re not supposed to go around diagnosing or curing people unless it’s literally causing them to be “disordered” because of it.

I’ve seen a claim that DID is actually caused by psychologists telling suggestible people they’re supposed to have DID, but maybe mr. SSC guy was joking about that one.