They don't really move to the left or the right. They just rule in ways that neither side really expects, but which is consistent with their judicial philosophy. Raich and Kelo outraged liberals, but it was entirely consistent with the liberal judicial philosophy that government has expansive powers. The liberal judges on the court, however much I disagree with their philosophy, are not going to change things on a case-by-case basis to get the result that those who supported them during confirmation may want. Same goes for right-wing judges. In the end, most will find a right to privacy in the Constitution, thus upholding abortion rights. Even if they do eventually figure a way around this, their commitment to states' rights will prevent any federal legislation on the issue. Therefore, pro-lifers think these judges have moved to the left, when in actuality they have done no such thing. They have merely upheld the very conservative view that government's powers are few and defined. Finally, there's precedent, which most judges are reluctant to overturn except in cases where doing so can be justified by their judicial philosophy. For example, Rehnquist believed that a single 5-4 decision was not binding at all, but several 7-2 decisions would be. Thomas is the only judge who does not believe in precedence. Scalia is very deferential to precedence, Kennedy and O'Connor even more so. So since most precedent nowadays is left-wing from the Democrats' domination of the judiciary from the 30s to the 70s, you'll find a lot of conservative judges voting in ways that respect precedence.