I've seen this too, actually. The "I know, so you just listen to me and forget any idea you had about it in your mind." I don't think anything generally causes it, but that it is found in their individual charts. Although I never thought of this before so it'll take me a while to decide what planet/object/aspect could cause this.
I also suspect they would have the same behaviour if it was another field or study.
It's kind of Astrological snobbery, isn't it? I do think some astrologers abuse the fact that astrology can be freely interpreted. They can say what they want, no one can prove them wrong because they can claim those are exceptions.
The snobs! :p
@Isa: I know I'm not a Sagittarius sun and I'm not going to claim that being a Sagittarius rising is the nearest thing. I like to think it is, but I have to be realistic... still a Scorpio sun... BUT, what I have noticed is that people with prominent Sagittarius influence often aren't taken seriously no matter what they say. I've seen it myself - hardly anyone takes me seriously, including myself sometimes, and I'm often patronised where there really is no need to.
You are very Sag as well, and perhaps that is why people bring you down despite all the studies you've done? I have to say, I've been studying astrology for longer than you but hardly as intensive!
And thank you for your vote of confidence in my answer, but I personally don't think I've really brought much into the situation. Thanks anyway :)