I've done the one you said about adding to a conversation that took place, maybe, HOURS ago. I have the utmost admiration for people who can follow me-----like, if I do it in a group of people and a couple of them look puzzled, and someone says: "She's responding to a conversation we had an hour ago"----I think that person is FABULOUS!! !
Anyway, I had a light-bulb moment just now. My eldest sister---whom I'm sure is an Aspie, yet undiagnosed----does the thing you said about saying things that have absolutely nothing to do with the conversation. I've always, in my mind, accused her of being selfish and bringing the conversation back to focus on HER / things SHE'S interested in---but maybe she is just "being Aspie"----or, maybe it IS being selfish not learning social etiquette, or whatever----cuz I think it's terribly rude. I think people who "dive-bomb" conversations may be being bored, uninterested, whatever, and that's their way of changing the subject to something in which THEY'RE interested-----which is, of course, socially unacceptable----but, we all have to continually learn, I guess-----however laborious it might be......