I have numerous theories about how this condition arises aside from being related to type, although many of them may stem from the root cause of the type. I imagine it is related to PoLR or weak although I've not made any strong record. Gender may play a role in how noticeable it is.
I would think that lack of practice talking is a key part of this - especially before 2 years of age, but then also somewhat significantly before 5 years of age. Perhaps having parents who have a different accent to the neighbourhood has an influence (I can't say this was true in my case, as I suspect my accent is still distinctly regional).
I wonder if having very particular, formal ways of speaking has an effect?
Another thing is having a habit of being very tight-lipped and almost gnawing at yourself ( which is how I am) - I always have a great fear of being overheard which also means I perhaps talk quieter than I should. I find that it is very important for me to try and make sure that a person is watching my lips (perhaps also combined with my hand gesture and general manner) otherwise they may not pick up what I say. Watching the lips must be a very important part of comprehending what people say - or at least, this is especially true in some instances.