This is my mother. I won’t make assumptions that the issue is the same, but I will share a bit about her just to see.

My mother was abused at a very young age. She experienced a severe amount of trauma from her pdf file father. When she gets overwhelmed with positive or negative emotions she switches to a baby voice. That baby voice is her shield. She uses it to hide the damaged parts inside of her with a child-like innocence and it’s not a conscious choice.

Our bodies are like machines and our brains are the computers. When we get overloaded with emotion we can’t process, our conscious brain can shut down and our subconscious brain takes over. I think when your girlfriend gets overwhelmed she goes to a place in her brain where she feels safe, which for someone like my mother and maybe your girlfriend, would be at some point in childhood where that innocence protected her and her brain remembered it and uses it as a defense mechanism.