Many young women picked this up from the Kardashians/Paris Hilton and now cannot "turn it off". We know this is not their 'natural' voice because if you ask them to make weird sounds and then a normal voice, then their normal voice sounds completely different than the sexy baby voice.
The weird thing is that the women themselves are surprised by this sound they're hearing and cannot easily reproduce it because the sexy baby voice is so deeply internalized.
Sounds rather similar to these involutory ticks.