I imagine it's a fear of not being heard, a consequence of being deprived attention by a primary caregiver in infancy
I'd have assumed the opposite, childhood encouragement of talking being the cause.
Otherwise I'd be a non-stop chatterbox, but I'm naturally a very quiet and introverted person.
I too was thinking along your lines. Like they were probably one of those kids that just constantly talked and interrupted people while the parents would just be like "Atta girl you tell them all about that!" Or even encouraging it "Oh hey sweetie, why don't you tell them all about the trip?" I went to elementary school with a few like that. They would always interrupt the teacher, and were constantly in trouble for talking, meanwhile out of school the parents just encouraged the behavior because everything the child said was just so precious. Spoiled brat more like. I remember when our music teacher told them to "Shut up, or go to the office!" The look of shock on their face was great.