I tried to figure out which mod is doing this with FNVEdit but to no avail. :sadvaultboy:
For what it's worth, this is not mod related. That happened to me several times during my first play-through, and I had no mods installed. That said, though, if I waited for about a minute, the conversation would seem to auto-abort. Either you are not waiting long enough, or this is a slightly different (but probably still related) game bug.