I don't think it's so much that the judges change ideology as that the debate du jour eventually reaches a topic on which the judge does not have a firm opinion. Roberts, for instance, was elected when conservatism was all about supporting the Middle Eastern wars, and I gather his views on health care were not scrutinized as closely. Similarly, the Nixon and Reagan judges were appointed when conservatism was about States' rights, gun rights, and anti-unionism; then, not too long after, the Court started handing down decisions like Lawrence v. Texas -- women's rights, gay rights, and student's rights suddenly and unexpectedly gained decent (not great) Supreme Court support. (Note to any aficionados watching, I am aware that my knowledge of US legal history is very spotty.)

While I tend to be aggravated by liberals' arrogant (and largely false) belief that intelligence governs ideology, I do think that liberalism persists in being less short-sighted than conservatism. I also think that's why the Supreme Court, with its permanent appointments, tends to be the site of key liberal victories, even following periods of conservative dominance.