I might be wrong but I always associated hippies with left-leaning, liberal politics. And I'm not sure where the association between the left and the anti-vax movement came from but I know it was a thing that was frequently made fun of. I even remember catching a Simpson's episode where they went somewhere and commented on how progressive/liberal it was (forget the specific word), then Marge asked a random woman if she vaccinated her kids and she responded "of course" then Marge said "and not TOO liberal".
Now that I think about it, maybe the political association of the original anti-vax movement was manufactured?