The answer to that question is Ayers. Obama and Ayers were both at Columbia U during the same period. Obama refuses to release any information about his grades or activities during that period.
Yes, I think it’s true that Ayers was the immediate one behind it. But who’s behind Ayers?
BTW, Barry and Bill seem to have lived within a few blocks of each other on the Upper West Side, and Barry won’t even reveal his actual address during those years...hmmm...