According to an American Spectator article many years ago, Kennedy was ready to overturn Roe v. Wade in Planned Parenthood v. Casey with the other four but somehow backed out at the last minute, and went with O'Connor and Souter. The reason has never been divulged
IIRC, the speculation was that he backed out because Byron White was not comfortable with using Casey as the vehicle to overturn Roe v. Wade.
I believe that Kennedy WILL vote with Thomas, Scalia, Roberts and Alito if there is a case that can reasonably be structured to overturn Roe.