I've noticed this recently among some of my American colleagues, and it's so obvious to me that at first I thought they were imitating an English accent. To my English ears, standard American pronounces the "t"s in butter and water as "d", and hadn't realised that some accents do have the glottal "t". I think it is mostly in people from the East coast.
spot-on in your analysis & as is mentioned elsewhere in the replies, I think it might be a definite east-coast influence on the rest of the US population as a result of video-based social media. at least, that's my running theory!
