In the infrequent case where conservative justices shift "left," the reason (I suspect) is that as they get older and as they recognize that their decisions have an impact on society at large (and not just the immediate litigants) they begin to take a look at people, rather than issues or positions. The shift is not to the "left," but toward pragmatism and humanitarianism.

My experience with the Supreme Court I practice before - in Rhode Island - is that they decide, based on their outlook, how they want a case to come out, and then find - or invent - the law to justify the result. So the law depends entirely on the judges' points of view - and if they find themselves actually caring, instead of pontificating, they can wind up doing the right thing.

That said, there are justices - Scalia, Thomas - who will never slip off their ideology. You have to suspect that people like that are sociopaths - they care about principles, not people, and that is about the worst thing you can get in a judge.