That is the ADHD. Constantly changing the topic to talk about something that happened to you once which you were just reminded of because of a detail in the conversation. Too much sharing of personal details with people I maybe just met. Telling too much to someone in spite of knowing that I should have shut up.

It has taken me ages to be fully aware that I do it, and when I notice I try to stop myself from derailing the topic being discussed. If I feel that it is truly relevant I cross my fingers or something like that to add it at the end of the conversation, but usually after a few seconds it loses the urgency and I feel I can drop it, or even forgetting what I wanted to add at that point. It is absolutely a social problem and I know people see it as a flaw of character, but it is slowly getting better. Being gently reminded that I am doing it helps because sometimes you just don't notice.

Of course if your acquaintance is on a too high dose of Vyvanse or has been wrongly diagnosed it can cause a bad case of motormouth, as all stimulants do...