I don't really understand what would have led Teddy Kennedy to become a Communist patsy, other than the fact that he seems to be both evil and stupid. Unlike many of those other people, the Kennedy family does not seem to have had a Communist background. Mobsters, yes, but not Communists that I have heard of. I would have suspected Jimmy Carter before Teddy Kennedy.
I'm still trying to figure that out as well. I think it's probably rooted in Robert Kennedy's shift towards the antiwar movement in the mid-60s. This occurred on several fronts. Vietnam was the most public expression of it, but there was also his relationship to Cesar Chavez, and his involvement with the left wing of the anti-Castro movement, which predated the Bay of Pigs. The Kennedys supported a left-wing faction of the anti-Castro exile community against a more conservative faction which was closer to Nixon and to Allen Dulles' faction of the CIA. I think the Kennedys' rivalry with Nixon and LBJ had a lot to do with it. Robert was planning to run against LBJ in '68, and Ted was a candidate to run against Nixon in '72 and remained involved in the campaign even after he dropped out following Chappaquidick, so I think it was partly a case of the Kennedys opportunistically aligning with Nixon's enemies, domestic and foreign. This became easier to do after Nixon initiated detente and Ted could legally travel to Russia; I think Ted was taking advantage of that opportunity.
I think that was the gist of it, and the '68 election was a big part of the motivation. Robert had started to break with LBJ on Vietnam a couple years before that, but didn't really publicly commit himself to an antiwar stance until about the time he entered the '68 campaign, after he saw that Eugene McCarthy seemed to have a shot running on an antiwar ticket.