This is entirely my own experience and may be coincidental (I can't be arsed to search to see if this is even a thing) but, listening to "Dr Drew" on love line in the 90's (late night radio), this stuck with me. He was talking about how often women who talk in infantile voices were often abused in their youth. The age/development period in which they were abused can lock them in to a part of that mindset.

Two women I have dated, did this on occasion. They would just drop into a juvenile voice when dealing with something that involved high emotion. In both cases, they confirmed that they had been subjected to physical in one case, and sexual abuse in the other. Re-enforced that one-off conversation I heard when I was little. Yes, the voice annoys me but had I not heard that one-off line from Dr. Drew, I would have probably not had the same level of empathy. The first time I brought up the Dr. Drew conversation, it caused her to break down in front of me.

Needless to say, it may not be something that they can control?