We all have other people inside our minds sometimes. Maybe our different personalities should even have names. I can call myself Sunny during the day, Mooney by night, Cloudy when it's gloomy outside, and Rainy when it's mucky and miserable. Sorry, just a trite little Split Personality sense of humor, I couldn't resist.
Or as the professionals call it, Dissociative Identity Disorder (DID)...
What triggers it? Leave that to the professionals to answer. Though Kevin Wendell Crumb was abused as a child and has not quite been the same since. Kevin suffers from the severity of DID. 3 girls and his psychologist are at the mercy of his illness. Only his doctor can comprehend the people who are inhabiting Kevin's mind and body at the moment weather it be Dennis, Barry, Patricia, etc. If I recall correctly, I believe Sybil suffered from the same disease.
Is it possible however, that there may be a more diabolical personality that is suppressed in the base of the abused mind? One that is practically animal like that is seeking retribution???